I see all manner of behavior in such situations. My wife would probably stop, and wave the person in, if she had time to slow down comfortably. Other drivers would lean on the horn continuously and speed up. My reaction is to get ready to brake and try to figure out what the person is about to do early enough that I have a contingency plan. Parking lots are where I see the most examples of people inching into a traffic stream hoping others will stop, before or after they can see who is coming. I also witness, among my own friends, the most extreme opposite attitudes about whom should yield in such situations. Usually, it's the other guy people think should yield.

I'm more likely to wave people in front of me on the bike since they'll usually pass me soon anyway. But if I were in the process of passing a stopped bus, I suppose I'd also be concerned about getting ahead of the bus before it starts up again.