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%.
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?