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.
Criminal #1: Oh shit it’s the one time, play it cool…
Criminal #2: Playing it cool, playing it cool… So why are they called the one time anyway?
Criminal #1: Because they only need to catch you one time fool!
Do you know when people are in jail when they're telling the stories of how they got arrested the first thing that they say is one time I was chilling with the homies and the popo's just pulled up on me and swooped everybody up. Every time they tell a story about when they got arrested or when they ran in with the cops they say one time, so when they see a cop they remember hey that's a one time story.
Hey dogg I'm going to tell you like this , one time I was chilling in the cut, slanging all that dope, A lot of cluccs and strawberries and the one tezzies came through with swat and hemmed us all up, had to do a mandatory 7 years down. Now I'm out and the one times ain't got Nathan on me. Let's go re-up lōc!