Mysterious Watcher

Buffy: We're having this thing at school...
Angel: Career week?
Buffy: How did you know?
Angel: I lurk.

—Buffy the Vampire Slayer, "What's My Line"

The chapter/episode is coming to an end. The Monster of the Week has been defeated, the Anvilicious Aesop has been duly absorbed, and the hero walks into the sunset with the Girl of the Week.

...and the shot pans out to reveal a mysterious character, wearing a mysterious trenchcoat, concealing his eyes behind mysterious shades and covering his face with a mysterious shadow, watching. Just... watching. Clearly, he's been there for a while. Observing the hero fighting. Assessing. Judging. And he's probably standing on top of a lamp-post while doing it.

Likely, the next episode will be all about him, revealing whether he's working alone or as part of the Ancient Conspiracy, whether he'll be The Rival, the Worthy Opponent, or the guy who's always just passing through when you need help, and probably - through the new character - expand the scope and size of The Verse.

Sometimes, a particularly mysterious stranger will watch the heroes for several episodes. Bonus points if he's seen noting something down on a PDA or notepad, or radioing information to The Omniscient Council of Vagueness through an earpiece or walkie-talkie. These days, some of them even use cellphones.

Mysterious Protector is a particular subtype of this - specifically, a 'benevolent' version. This guy just as often turns out to be a foil, rival, or downright antagonist. This trope is sometimes also combined with Chekhov's Gunman. The Mysterious Employer is another subtype: instead of being "benevolent" per se, they direct the heroes in some way.

Not to be confused with The Watcher, whose explicitly-stated role is to observe events from afar (and whose implicit role is to pop in to offer advice to the protagonists).

Examples of Mysterious Watcher include:

Anime and Manga

In Cardcaptor Sakura, both Shaoran Li and Kaho Mizuki are introduced this way. Kaho is particularly noticeable for wearing shades during her enigmatic first appearance, and never again.
Bakura in the Yu-Gi-Oh! anime spent some time lurking in the background before he actually started playing a role.
Madoka, Arisu, Kaede and Sai in Angelic Layer. In the manga, they were even shown in silhouette. The English VAs for Tamayo and Koutarou lampshade the practice in an audio commentary, complaining that "soon, they'll have more lines than we do". Shuuko later takes up this role.
Caren in Mermaid Melody Pichi Pichi Pitch. Later, Sara indulges in it once in a while.
During the Makoto arc of Kanon, Amano Mishio stalks Makoto and Yuuichi before making herself known.
The invisible presence of Hanyuu in early chapters of Higurashi no Naku Koro ni is a tragic version of this, seeing how much of events is driven by misunderstanding and paranoia.
- Frederica Bernkastel may also count, seeing as she writes poems about the series' events without actually interfering.
This is how it was confirmed that Johan was still alive on Yu-Gi-Oh! GX. Edo also liked doing this while he was playing Batman.
Xelloss is introduced this way in Slayers NEXT.
Autor in Princess Tutu is usually just a Chekhov's Gunman that "just happens" to be in the library the same time as Fakir, but in two episodes he's shown just...watching Fakir. His eyes are covered up by Scary Shiny Glasses until the episode that he gets a line outside of "Will you please be quiet?!"
Oscha in the first episode of Final Fantasy Unlimited.
Naruto introduced Itachi and Kisame this way.

Comic Books

The Ninja, Toyata, in Y: The Last Man. For a long time her whole schtick was just watching the heroes through binoculars. Then her schtick became screwing over the heroes. Her first appearance was kind of both, though.
The Clone Saga of Spider-Man has a wild number of different Mysterious Watchers, to the point where even The Life of Reilly mentions it.
In DC Comics's different titles in the months leading up to The Crisis on Infinite Earths, the Monitor was depicted this way. He was not clearly seen, and he was literally "monitoring" the activities of different super-powered beings?sometimes intervening in various roundabout ways to put them in jeopardy to "test" them. He was made to look very sinister.
Star Wars Legacy handled Azlyn Rae's introduction that way.
Capes featured a shady young man constantly spying on Bolt and muttering about getting revenge for his father. "Soon, Bolt... soon." When he meets a nice girl while stalking his prey and gets engaged to her not long after, he amends his mission statement to "...Later, Bolt. Later." Who he is and what Bolt did to his father are never explained.

Literature

The Dread Pirate Roberts, in the original novel of The Princess Bride, is doing exactly this in his first appearance in the story.

Live-Action TV

This is how Angel first appeared on Buffy the Vampire Slayer, despite not being Buffy's actual Watcher.
- Spike sometimes tries to do this, but has a tendency to get bored and try the Indy Ploy instead.
Ruby on Supernatural spent some time mysteriously stalking Sam before revealing herself.
- At the end of the season 5 finale, after Sam is sucked into hell and Dean has gone to live with Lisa & Ben, we see a shot from outside the house: Sam is watching them eat dinner.
Sela on Star Trek: The Next Generation.
The Watchers from Highlander the Series.
In The X-Files, the Cigarette Smoking Man spent much of Season One standing in the background looking mysterious and occasionally filing away some vital piece of evidence that had gone missing from the crime scene.
At the end of the second series of Robin of Sherwood, after Robin has been killed, the outlaws see a hooded figure who looks like Robin (and at this point is presumably still played by Michael Praed). At the beginning of Series 3 he is revealed to be Robert of Huntingdon (Jason Connery), who of course becomes the new Robin.
Dwight Dixon in Pushing Daisies has a tendency to lurk in the background, watching, plotting, usually carrying some sort of firearm...
- After Dwight Dixon dies, the episode The Norweignians sets up another Mysterious Watcher... Ned's father, who abandoned his son as a child. Cue cancellation. Goddammit!
The Observer (AKA September) from Fringe appears in every episode, sometimes only for a few seconds and usually without the characters noticing. The episode that revealed him was a subversion of this trope as he got punched in the face for his trouble. He and his fellow Observers are a group of men with advanced technology who observe key events in history for unknown reasons, but are bound to not interfere.
HRG, from Heroes. He later becomes a central character after he's revealed as Claire's father, but for the length of the pilot all we saw was a mysterious guy in horn-rimmed glasses.
Tower Prep features one in its pilot.
The aptly-named Watcher (now using the pseudonym ~~Will~~ Joshua) from No Ordinary Family.

Other

The Gatekeeper in Knowledge is Power. So far, his intentions seem benevolent, but it's hard to tell when so far he's mostly just lurked in the background.

Professional Wrestling

This is how Raven was introduced to WCW: he'd show up during certain people's matches and take a front row seat, and just watch mysteriously.

Video Games

Gamov from the Xbox remake of Ninja Gaiden, complete with coat, shades and high vantage point. He does introduce himself to Ryu quite early, but otherwise spends his time as one of these characters is meant to act.
Final Fantasy Tactics A2 has this for one side mission where you are the mysterious watcher. In the mission, your objective is to simply watch a level 99 Paladin and a level 99 Master Monk duke it out till one of them is victorious. You can only send out one unit to watch the fight and it is likely that your level won't match theirs if you decide to try and attack them.
Raven from Tekken 5 is introduced as one of these in the opening animation. And he's still one by the end of the game, because even if you play as him you never find out who the heck he's actually working for.
The G-Man from Half Life is a textbook case. He even manages to keep up the schtick after meeting the hero face-to-face, since even then he says a lot but reveals exactly squat.
Throughout the F.E.A.R. games, there will be moments where, if you're quick enough, you can spot some thing watching you from corners or the ceilings of rooms, before sprinting off in a barely-visible blur and disappearing into an air vent or behind a grating. Sometimes, these are just the Replica Assassins that have been quietly tailing you throughout the game and feeding your location to their comrades. Other times, they're something much, much worse....

Web Comics

The TicTocs (also called the thousand eyes) from Gunnerkrigg Court have been watching since the very first chapter. We still don't know who they work for, or what their goals are.
The Mysterious Watchful Presence watches mysteriously at the end of Chapter 1 of Everyday Heroes.

Web Original

The Watchers in Lonelygirl15 and Kate Modern.
- Subverted in lonelygirl15 series 3: after the death of Bill Porter, his Shadow starts following the gang, appearing in the background of several videos, and leaving threatening messages. He then runs out into the road and gets run over, ending that plotline abruptly.
The Slender Man tends to do this. And then he gets a bit more up close and personal.

Western Animation

There was an episode of Batman: The Animated Series where, after teaming up with a Femme Fatale to fight a supervillain, the episode ended with Ra's Al'Ghul watching from afar via a Palantir Ploy, and saying something like "Most impressive, Detective" - and then later becoming the enemy in a two-parter. It's not Batman's fault that we all know Ra from his appearances in all the other series, movies, and comic-books. To be fair, this was the first appearance of Ra's outside the comics, and even after Batman Begins, most non-comics fans don't know who he is.
Mirage from The Incredibles was introduced doing exactly this.