As far as stopping by without an appointment? No, please don't. If you drive by and no one appears to be there but me doesn't mean that someone isn't GOING to be there in the next few minutes. That would look VERY bad to the person arriving for their appointment!
If you forget something? If you JUST left and turned around to come right back, I would be okay with that but if 20-30 minutes have gone by, it would probably be better to call first.
I would NEVER call nor email gentlemen to try to get appointments or anything else. The guys will contact ME when they are ready to make an appointment.
Seeing each other in public places? Well, it is ALWAYS okay to say hello to me no matter where I am or who I may be with. It is absolutely never a problem at all. If I see a gentleman that I have seen BCD, I would NEVER approach nor acknowledge him even if he appeared to be alone. You never know if he had family around or not.
The things that you THINK would be common sense sometimes go by the wayside when you are caught off guard. Just be careful out there!