Why the Specificity Is the Threat
The signature is not scary. The detail is.
Two kinds of "I saw you": "Someone saw you doing something bad" is a threat - it has a payoff. "Someone saw you doing something *oddly specific*" is different - it is just a detail, and details are harder to explain away than threats. You cannot argue with a detail. You can only confirm or deny it, and confirming it is the whole punchline.
The detail you pick: it has to be small enough to be embarrassing and big enough to be memorable - the 2am reorder, the incognito search, the "seen at 1:47" on a message you did not actually read. The smaller the detail, the more the listener believes a watcher exists.
Why it replays: the format is a guessing game about who "A" is - a real person, a bot, a friend in on the joke, a stranger with too much time. Every replay is a vote on which one it is.