The police. So called because you look one time at them, as doing a double-take might attract attention.
See Sir Mix-a-Lot's song "One Time's Got No Case".
The cops. It's correct that it means looking more than once can draw attention. But the actual phrase comes from someone saying "one time" to let the other passengers or friends know that they have now seen the cops and are claiming the first look, and no one else should now turn around because two or more people looking increase your odds of being stopped by more than 50%.
"One time, man. Don't turn around!"
Amongst other terms, the polite thing to yell when you see a cop cruising around, especially slowly.
"One-time! One-time!" (Then walk briskly in the opposite direction.)
the police, meaning you've got one time to run, or "eat yo shit nigga"
"Fuck, here comes tha onetime!"
Yo! One up holmes, it's the one-time!
Police. See #1. You look at them one time. If you double take or look too long you might attract their attention. In doing so, with a cop with nothing better to do and if in their minds they suspect an ounce of fishy shit going on with you, expect to get followed for a bit so they can check your plates, pull you over and/or get talked to to see what's up with you.
From the movie Street Kings
Coates: Man, we got the one time up in here. Motherfucking po-po. Is there any square cops out there? Or is everybody out for theirs?