Next time you go to your local stop-n-rob and have a few minutes, watch. Most people will drive up to their parking space. They will either not see or ignore anyone they see sitting in other parked cars - even gang-attired "yutes" (as they say in New York) sitting in the drivers seat of a running "yute"-type car. They will walk in to the store without so much as a glance anywhere but straight ahead of them. Or try standing just off the aisle by the cashier where a robber-in-waiting would stand and see how many people even notice you there. It's not that they aren't suspicious of you - they probably won't be if you aren't giving off I'm a nervous suspicious guy here to rob the place signals - it's that they won't even be aware of your presence.
Now compare this to the behavior of the best cops that you know. They don't slink around in a combat crouch; they just look around as a matter habit. They don't know any secrets that you don't. They don't have any abilities that you lack.
They just have a habit.