Disposable Vagrant

""You need subjects for your Servant project, and I can provide them. Beggars, prostitutes, street scum - those who will not be missed by anyone of consequence...""

- Sheriff Truart, Thief 2: The Metal Age

Vagrants, the homeless, drifters, runaways, society's castoffs; there's many, many names for them, none of them nice. Most people look away and pretend not to hear them when they ask for change, if not outright disdain and mistreat them for the crime of having been left behind by society-- or having left society behind. With so many moving around, who would even care enough to notice the bum who is usually in the street corner of Trope and Tan every morning has gone missing?

And that is precisely what the Serial Killer, Mad Scientist, hungry monster and The Mafia are counting on. These exceedingly vulnerable Innocent Bystanders are frequently killed and abducted for fun, spare parts, to sate hunger or for a spare dead body. They can be sure the police won't push the investigation too hard; at least not until a rich, beautiful (and preferably female) character is kidnapped or killed.

If anyone does notice, it'll be The Hero, an aid worker or beat cop who notices familiar faces disappearing. Usually though, quite a few will die before the culprit is ever caught.

Compare Disposable Sex Worker, for a profession at risk of violent crimes, and Kill the Poor, for more high profile slaughter of the impoverished on a much wider scale. Contrast Missing White Woman Syndrome.

Sadly Truth in Television.

Anime and Manga

 * In Durarara, Yagiri Pharmaceuticals hires kidnappers to abduct drifters and illegal aliens who won't be missed to be used in their medical experiments.

Books On Trope
"Experiments requiring a human test subject shall be performed on kidnapped anti-social bums who live alone in large cities, not someone who people will notice the disappearance of, like a female student at the local high school."
 * From Things I will do when I become Evil Empress, Tips for Evil Geniuses and Scientists:

Comic Books

 * One issue of The Flash has him team up with the Pied Piper to stop a Serial Killer with a virulent hatred for the homeless.
 * The short-lived Marvel Universe comic series The Order had "zobos", down-and-outs turned into zombified cyborgs by the Big Bad.
 * Runaways. It seems that The Pride preferred to use female teen runaways for their annual Human Scarifice.

Film

 * In Hard Target the Big Bad is the head of a hunting business which allows rich men to hunt homeless or down-on-their-luck war veterans. Homeless or down-on-their-luck so no one cares, war veterans to make it interesting.
 * Classic 1980s B-movie C.H.U.D. plays this straight and also inverts it. Several homeless people start turning up missing, but only two workers at a soup kitchen seem to notice or care about this. Later, it's revealed that they're the victims of other homeless people who have been mutated into killer monsters due to exposure to illegally dumped toxic waste and have begun to munch on people who aren't deemed disposable by the powers that be. Along with being a horror movie, the film also functions as a Take That against Reagan-era policies toward the homeless and mentally ill.
 * In The Blob, the titular monster's first victim is a vagrant whose name we never learn and who exists merely to show the destructive effects of the blob.
 * In Rat Race, when a heart meant for an emergency transplantation gets damaged, the character responsible for delivering it suggests looking for a 'drifter', killing him and cutting his heart out as a replacement.

Literature

 * A Stephen King novella called Apt Pupil about a teenager who kills homeless people.
 * The villains of the novel Rainbow Six use homeless people as test subjects for their plan. The book spends quite some time following them as they have no clue what's going on.
 * In the sci-fi book Parable of the Talents, the Christian America cult started out with putting vagrants in their concentration camps because nobody would notice or care.
 * CS Lewis' Out of the Silent Planet opens with mad scientist Dr. Weston debating whether to kidnap a homeless retarded boy or his best friend from college for his rocket expedition.
 * Michael Stackpole's Wolf & Raven short story "Fair Game", set in the world of the Shadowrun Tabletop Game. The Pacific Northwest Hunting Club is made up of wealthy people who hunt humans once per month. They choose their prey from among the SINless of Seattle, who are not part of the system and whose deaths are not even investigated.

Live Action TV

 * On an episode of Barney Miller, a homeless man comes to the station to report that several other homeless people have mysteriously disappeared. Harris goes undercover as a homeless person and discovers they had all been kidnapped and forced to work as slave labor on a farm in North Carolina.
 * In the Buffy the Vampire Slayer episode "Annie" the Monster of the Week takes street kids into a hell dimension and works them for many many years, which is equivalent to only a few days on Earth. He spits them back to Earth just before they die of old age.
 * In the new Doctor Who the homeless were rounded up to become the new Cyberman army in "The Age of Steel".
 * We don't learn if this actually gets noticed or not, as the first thing the first batch of Cybermen do is start rounding up everyone else to convert.
 * Later subverted in the final scene of "Day of the Moon," when a Creepy Child approaches a homeless man and says, "I'm dying. But I can fix that. It's easy, really... see?" She holds out her hands, and we wait for her to suck his life out in some gruesome fashion... but instead.
 * In the old series, The War Machines featured a vagrant who had slept in an empty building the villains had taken over. They kill him.
 * But the most notorious Doctor Who example is the incomprehensible Pigbin Josh from "The Claws of Axos".
 * In Fringe, a Neo-Nazi tests out his ultimate weapon near a homeless person to see if he dies.
 * In the pilot episode of Robo Cop the bad guys were harvesting homeless people for their brains, and only got caught when they moved from homeless to an OCP company secretary.
 * At least one murder on Law and Order Criminal Intent was hoping that the police wouldn't look too hard at a bunch of dead homeless people turning up. Big mistake.
 * In The Cape, several lived in the train yard where Vince "died." They're being arrested on trumped up charges rather than killed though.
 * In The Invisible Man, the Doctor who helps Hobbes has been harvesting brain cells from the homeless.
 * Jimmy McNulty from The Wire fabricates a fraudulent serial-killer case around or inverting this trope using vagrant corpses in order to atract media and political attention and divert funds to real police work. He even "abducts" one live vagrant to futher drive the point home.
 * A Mad Scientist in Lois and Clark states she always uses people who won't be missed for her experiments... and is very distressed to learn the latest subject was undercover.
 * One episode of The Pretender featured a research doctor who was testing his resuscitation drugs on the homeless-- which meant stopping their hearts.
 * In the Tales From the Crypt episode "Mournin' Mess" a secret society of ghouls start a funeral service for homeless people (that they kill) so they can eat the corpses without the risk of being caught gravedigging. Slightly subverted since it's implied that the ghouls have to bribe city officials to keep the police from investigating the violent murders too closely.

Tabletop RPG

 * Call of Cthulhu supplement Cthulhu Companion, adventure "The Rescue" — a group of werewolves deliberately targets derelicts; by victimizing those without friends or money, they ensure that the police are not unduly concerned.
 * This was the plot of Wolfen.
 * Dungeons and Dragons:
 * In the supplement Volo's Guide to Waterdeep, a mimic monster took the shape of a statue in a city square, killing and eating drunks on dark nights for two years before an investigation was launched and it was discovered. A nearby sewer was found to be filled to a depth of 60 feet with human bones.
 * Supplement RR3 Van Richten's Guide to Vampires says that vampires often kill poor people whose loss won't be noticed.
 * Module A Darkness Gathering. A Thieves Guild leader (who is under illithid control) orders her underlings to kidnap vagabonds, beggars and visitors to the city. She has their heads cut off and given to the illithids so they can eat the victims' brains.
 * Module A4 In the Dungeons of the Slave Lords. The lacedons (aquatic ghouls) who live offshore from the city of Suderham sometimes sneak into the Scumslum section and grab a derelict for dinner, who likely will never be missed.
 * In Shadowrun, the Universal Brotherhood was a cover organization organized by insect spirits, insect shamans and their victims. Many UB facilities were set up in run down areas and ostensibly tried to help the homeless, while actually arranging for them to be possessed by insect spirits.
 * In the supplement Double Exposure, an unholy alliance between the Renraku Mega Corp and insect spirits exploits homeless people in Seattle, testing experimental medical products on them and changing them into monsters.

Video Games

 * Blood Rayne 2 had homeless people (and sex workers) being kidnapped to be turned into a blood fog (the shroud) that protected vampire from sunlight.
 * The villain of the second Thief uses the homeless to create his army of Steampunk cyborgs. It is explicitly stated that the nobles don't notice or care, and the victims are being arrested by the Watch, shipped and converted in small groups so that it goes unreported.
 * Vampire Bloodlines:
 * One of the Plaguebearers in a sidequest used the homeless to spread his infection. The other one used prostitutes.
 * Andrei the Tzimice used, among others, homeless people as raw material for his experiments...and his home improvement projects.
 * In Portal 2, the recorded messages of Aperture Science founder Cave Johnson showed his first batch of Mad Science tests were carried out using the 'best and brightest'. In the second set of tests from years later, his recorded message introduces himself as the man you might know from the 1968 missing astronaut Senate hearings. He is now offering his new batch of test subjects 60 bucks (60 more if they let him "take them apart and install gadgets in them"), and mentions how much nicer the waiting room must be compared with the park benches the test subjects were sleeping on before. In the third set of recordings, he's apparently discarded this idea and moved on to testing on his own employees, including himself.
 * They're allowed to lurk around the waiting room for a whole twenty minutes before and after testing! Luxury!
 * In Assassin's Creed, the "patients" of Garnier's hospital are mostly beggars, prostitutes, madmen, and other homeless taken from Jerusalem by Talal's slave ring. Garnier's plan was to use his drugs to cure their addictions and insanities, followed by training and conditioning them as soldiers, to turn them into loyal warriors in the service of the Templars.

Web Comics

 * In The Last Days of Foxhound, Revolver Ocelot claims that Moscow has the best hobos for the purposes of torture (and invites Sniper Wolf to join him).

Western Animation

 * Both Batman the Animated Series had episodes about homeless people being abducted for use as slave labor. To them, no one seems to care, but obviously Batman is on the case.
 * A villain-of-the-week uses homeless children to carry out a series of thefts in the city. Batman is not happy about it, to say the least.
 * In an episode of TMNT a super villain called the "Garbageman" abducts homeless people turning them into slaves, referring to them as "human garbage". The police haven't noticed the abductions or perhaps don't even care.
 * In Disney's Gargoyles, Dr. Sevarius uses Maggie, a homeless woman, in his mutation experiments because nobody will notice she's gone. All of the mutates but Talon (Derek Maza) have no known families.