I ran into someone who had interviewed at my company, we were bot on an inter-airport shuttle in I think Berlin. He was based in CA, I was in MA, and we weren't really in the same industry (his knowledge base was tangent to what we did, thus the interview), and not traveling for the same event.

Have had a handful of similar scenarios, seeing someone I know when we are in a far-away random place. I think it had to do a lot with I was traveling frequently at the time (200K miles/year), to all kinds of random places.