Play it safe and do not approach them. Unless it is someone you know so well and it would be ok.
For instance, the funniest one that ever happened to me: I had just finished the session with one of my regs (super nice and sweet guy).
It was my last for the day, so I cleaned up, etc, got dressed, even needed to carry some things down to my car. Decided to stop by the store on my way home. As I walked through the door to enter, there he was walking out at exactly the same time, we came face to face, lol. So it was just a quick, "Oh hey there! See ya later!"
And we went our separate ways, lol.
However, I have had the other when you are already out and you happen to see them from a distance - I recognized them, they me, but we kept our distances and did not approach each other. I swear Houston is the biggest small town!