Some kind of civil servant knocks on the door.
You get spooked by a noise and it turns out to be the cat.
You and two friends find yourself in “the spooky old house on the hill”. For what ever reason you decide to explore it. You decide to split up/explore separately. All of this is pretty bad,
but…
If one of you is a pretty teen aged girl in a bikini or a cheer leaders costume…
stick a fork in… your done
A car pulls up alongside you and some huge dude tells you to get in.
A cough
On the other side though, if you are the monster lurking in the old abandoned house and a harmless-sexy-girl type like that shows up, when you leap out to attack her she’ll probably throw you though a wall or pull out a big blade or guns and slaughter you. Or some large fellow will burst out of nowhere and do it. At the very least peek outside and see if there is a van or a few cars with more teenagers/early twenties lurking outside but not entering; those are potential sidekicks and a warning sign that a Protagonist may be nearby.
It’s a dark and stormy night…
“Hello, 'Plant? You know that one phone call the cops let you make?
…This is it.”
“Who’s bra is this” 
or
“Hey guys, watch this!”
Sometimes prefaced with, “Hold my beer a sec.”
Even worse: “Hold my beer… watch THIS!”
Aaaaah simulpost!!! 
Chris Hansen walks in the room.
The head of the project starts the briefing with “This is the most advanced (fill in the blank) ever made by man!”
Clowns. Fuggin’ clowns.
A big red button.
Dramatically cocking a gun.
Sex (teenagers only).
Being naked at times that nakedness would be unusual.
Anything glowing that wasn’t glowing before.
Someone said it before, but phone calls late at night. Nobody every calls you at 2 am to tell you that you won the lottery.
For that matter, winning the lottery.
You and your wife hire a nanny that seems perfect and totally overqualified for the job.
You’re visiting your fathers grave and your brother is acting like a douche.
“Hi. We’re from Headquarters. We’re here to help.”
They’re coming to get you, Barrrrbarrrra . . .
“So, have you told anyone else about this?”
A close family member works for the bureau that only monitors freak, unusual weather events…
Quite a lot of mad cackling laughter seems to come from that nearby castle on the hill…
You have just been made welcome by Norman Bates.
You stumble across a very big, thick book covered in strange inscriptions and sealed by a lock and hasp…
Your daughter just talked to you using the voice of Satan.
You tell people about the fascinating chat you just had with the old man who lives in the cottage a mile or so back. And they tell you that there is no old man, and no cottage. At least, not counting the one that burned down exactly one hundred years ago to the day…