Beneath Suspicion

"Brick, down in the gutter, had dropped below even that horizon. No wonder Chrysoprase's shakedown hadn't corralled him. Brick was something you stepped over."

—Terry Pratchett, Thud!

A character who is clearly linked with all the victims of a crime spree is inexplicably not even regarded as a suspect by the detectives until halfway through the final act.

Often seen in conjuction with Never One Murder. More often than not ruined in live action by a familiar face.

Pretty much endemic in murder mysteries, especially British ones like Taggart, Midsomer Murders, et al. Done properly, the writer will be able to convince even the audience, who are Genre Savvy enough to regard everybody with suspicion, even the detectives.

Source of The Butler Did It. Also see The Dog Was the Mastermind.

Examples of Beneath Suspicion include:

Film

Played with in the first Scary Movie, as the killer is "posing" as mentally handicapped.
The obvious candidate in the 2007 horror Drive Thru is the owner of the drive thru chain and the father of the kid died in an accident when he was 18. The police only suspect him 3 quarters into the movie, but this is subverted as they are Wrong Genre Savvy: they're not in a normal murder mystery, the killer is the ghost of the dead kid.
Played with (along with everything else) in Hot Fuzz; whenever Nicholas voices his suspicions of Simon Skinner, people respond that he runs the local supermarket, as though that puts him beyond all possibility of wrongdoing.
- Well, mostly, it's because the police don't believe that any murders have taken place at all, as they have all been set up to look like accidents Except for the Chief, who is one of the murderers himself.

Literature

In the Revenge of the Sith Novelization, Mace Windu states that the only reason Palpatine (the actual suspect) is above suspicion of being the Big Bad infiltrating the Senate is because he already rules the galaxy.
Justified in Parfum (the book, not made clear in the film) because Gaston has no personal smell he almost cannot be remembered and slides under everybody's radar.
Arsenic and Old Lace: Who would ever suspect two nice old ladies?
The killer in Below Suspicion was in a prison cell when the murder was committed.
Remarked on by author John Dickson Carr in an essay on the Fair Play Whodunnit: never remind the reader that a suspect has an airtight alibi, or he'll immediately be suspected. Treat it as such a given that it never occurs to the detective (or the writer!) to suspect Joe because Joe is so obviously innocent.
Harry Potter:
- Professor Quirrell in Harry Potter and the Philosopher's Stone. Ginny Weasley in Harry Potter and the Chamber of Secrets. Deconstructed or something in Half-Blood Prince, when Harry's spying on Draco Malfoy has him convinced that Malfoy is a Death Eater and responsible for lots of the life-threatening mischief at Hogwarts that year. Everyone he talks to finds this very far-fetched, because Malfoy's just a teenage student and not even a particularly competent one. Of course, he turns out to be right.
  - It's also played straight in the same book. Harry never once suspects the correct person of being the Half-Blood Prince and has to be told who it is. this is despite his habit of suspecting Snape of anything and also the (once common) tradition in British schools of school teachers keeping their favourite text book in the book cupboard and it only ending up in the hands of pupils if they're desperate (precisely because of how moth-eaten and scrawled over these books often were). Apparently Ron and Harry were familiar enough with their own education system to fight over who didn't get the old book but weren't familiar enough to associate that book with being the teacher's. As a result, the one time Snape should have legitimately been one of Harry's (or at least Hermione's) first suspects was the one time he inexplicably wasn't suspected at all.
  - On the other hand, Slughorn had taken over for Snape as potions master that year, so they only had Slughorn to relate it to.
- Also used in Harry Potter and the Prisoner of Azkaban. You can't get much more Beneath Suspicion than someone's pet rat.
- Animagi seem to like using this reasoning, especially unregistered ones. In Philosopher's Stone, Professor McGonagall spends all day spying on the Dursleys in the form of a cat. Later on, various other animagi try using the same reasoning with varying degrees of success such as Sirius trying to get away with using his dog-form and Rita Skeeter obtaining her stories by turning into a bug.
Lots and Lots of Agatha Christie novels. The most notable example would probably be Crooked House, in which the murderer is a psychopathic child which no one in the book, nor the reader for that matter, would have ever suspected. Caused quite a stir in its time, too.

It gets to the point that the character(s) that have absolutely rock-solid alibis are often the ones responsible. Examples include Lord Edgware Dies (she was at a party with friends), Death on the Nile (one had been shot in the leg, the other with a nurse looking over her) and Murder in Mesopotamia (he was on the roof while the victim was downstairs).

The murderer in Tamora Pierce's Shatterglass ends up being a prathmun, a member of the Untouchable caste, considered so low and degraded that to even acknowledge his presence requires being ritually purified afterward.
Deliberately invoked by John Kelly in Without Remorse when he goes on his Roaring Rampage of Revenge while disguised as a bum. Had he not accidentally walked onto the scene of a totally unrelated mugging and left behind a wine bottle with no fingerprints on it, the police might not have realized how he was operating.
In Dune, Dr. Wellington Yueh is the obvious suspect to be the traitor who will betray the Atreides to their Harkonnen rivals. However, he has supposedly been the recipient of Sukh mental conditioning, guaranteeing that he can never voluntarily take a human life. Therefore he is able to fool even a Living Lie Detector who is specifically alert for signs of potential treachery. In other words, he is set up as a Red Herring Mole to conceal the fact that he is actually The Mole.

Live-Action TV

This is used in an episode of Sherlock when the killer was a taxi driver.

Sherlock: This is his hunting ground. Right here, in the heart of the city. Now that we know that his victims were abducted, that changes everything. 'Cause all of his victims dissapeared from buisy streets, crowded places, but nobody saw them go. Think! Who do we trust, even though we don't know them? Who passes, unnoticed, wherever they go? Who hunts in the middle of a crowd?
Watson: I dunno, who?
Sherlock: ... I haven't the faintest. Hungry?

Tabletop Games

In one Paranoia adventure, when a robot claims to have video footage exonerating the PCs, the gamemaster is advised to maintain this trope: "Don't go 'heeeeeeey, there's a data port right over there, wanna try it?'. Wait for the PCs to ask if there's a data port nearby, then casually say 'oh yeah, there's one over in the corner'." When the robot is hooked up, it restores the previously-crashed Computer.

Video Games

Dee Vasquez and Acro in Ace Attorney. Somewhat justified by the fact that the investigators didn't even know Vasquez was near the murder scene until the very end of the first trial day, and Acro is in a wheelchair. Acro basically even says, "I'm in wheelchair, you jerk, how could you accuse me?!"
- Also the true head of the smuggling ring in Investigations and culprit of both 5-5 murders is the sweet, self-effacing Manipulative Bastard Quercus Alba. Despite being the ambassador from KG-8 to the present, nobody thinks to investigate the guy who can barely walk even with a cane.
- In Ace Attorney Investigations 2, it takes until the very end of the fifth case for Edgeworth to realize that maybe the best friend of the first case's victim might be somehow connected to events. Pretty justifiable, as in this case 'events' meant multiple kidnappings, manipulating two high-level law enforcement officers into committing murder, and hiring an assassin to kill a president. Said best friend was a clown.
Persona 4: No one but the most Genre Savvy could have seen Adachi as the murderer. But you, the player character, are also called under suspicion with your snooping around but the main detective can't believe that the guy who's helping him raise his daughter would do such a thing.
In a meta example, Kalas in the first Baten Kaitos game. Few players would expect to be betrayed by the main character.
- In an even more meta example, the sequel has the player themselves (unknowingly) lying to Sagi and co.

Web Comics

Strays sets out to invoke this by having Meela pose as a servant.
Impure Blood. If she hadn't told him, he would never have guessed she was not a servant.

Western Animation

Scooby Doo plays this trope straight constantly during its early incarnations, although they begin playing with it in later series and spinoffs. In the original series, the one character the gang briefly meets early on in each episode disappears and is never seen again... Until the monster is captured. He usually tries to make himself extremely helpful during the brief time he's seen, which is another hint.
- Double subverted in one episode, where the kids meet a creepy old man who tells them a creepy story of a haunted house, then disappears. They spend most of the episode trying to catch a headless ghost in said haunted house, only to find out it's the inheritor of the house (a person they've never seen before), trying to keep treasure hunters away until he can recover his grandfather's fortune. The next moment, a masked burglar wearing a bedsheet on his head breaks into the house. They catch him and guess what? He's the guy they met in the beginning.
- In another episode, they are alone for the first half without meeting anyone. This one has no disguised villains, just a malfunctioning robot and an inventor trying to repair it, and his wife, who doesn't like robots, and only appears at the very end.