Y'know, those small banks you may have come across and even attempted to rob? I say attempted because if you've had the same luck I had, there's nothing to be gained from it besides potentially alerting the police.
I had assumed they were a scene for a mission, but I'm getting pretty close to the end stretch now--Michael's just had to save his family from Devin's PMC guys invading his house, and I'm now moving on The Big Score--and to my recollection there's been nothing going on with them as of yet. Maybe they're to be mini bank heists for GTA Online?
Or maybe they can be robbed and I'm not doing it right... Though the clerk's never facing you in any case, and even when you are able to try and hold him up, he'll just cower onto the floor and you still can't lead him to the safe or anything.