What's a girl to do...

I've been dating for years now but I've experienced a new phenomenon in 2012, most recently on Halloween, something that I can only describe as awkward haunting (for want of a better phrase).
In the past, I would go on a date or two, realise it was going nowhere and never see the boy again.
This year, something different has happened, I seem to be bumping into boys I have been on a date with at the most unexpected times all over London, months after we last met.
I'm not 100% sure how to handle this situation and not exactly proud of how I have handled it so far:

1) the first time it happened was like a scene from a Bollywood film; I was late to meet a friend for lunch, we had decided to meet near the tube station, however, I got there and she was nowhere in sight. It had started to rain, i called her and she directed me to where she was, i followed her directions and ended up in a leafy square that was seemingly deserted...apart from one person, that is. The boy who had stolen my heart just a month earlier and then disappeared, had entered the square a few seconds after me from a different entrance. We were on course to cross paths, so what did I do? I turned around and strategically hid behind some trees! My reasoning, I was completely unprepared for such an encounter. Here was the first boy who had made me believe that I could fall in love in years, I had thought of him many times and wanted to see him again - but not when looking like a drowned muppet. He walked on to wherever he was going and then it hit me, I had just lost a great opportunity. I haven't seen him again.
2) the second time, in what seems like a recurring theme in my life, I was late for a meeting and rushing to the venue. I stopped at a busy junction, waiting for the green man to appear. In a world of my own and worried about walking in late to a round table discussion, I crossed the road and saw someone walking towards me and smiling. I assumed that he was smiling at someone behind me but as we crossed each other I realised he was a boy that I had been on two dates with, which had ended acrimoniously. There was no-one behind me, he had indeed been grinning at me. What did I do? I blanked him. This is something that I'd never usually do in my dealings with people. I felt bad about my rudeness.
3) the third time, on Halloween night, I was walking back to the tube after a work event, down a busy central London shopping street. It was a cold and wet evening so I had my hood up. walking towards me was a boy I had been on a couple of dates with earlier this year. I'm pretty confident that he didn't see me as my head was covered, I felt relieved.

What is the best thing to do in these situations? Do people cross our lives for a reason or are these just a series of random occurrences, which deserve no afterthought? 11 million people live in London (I think) and we see the faces of many many strangers each day. Bumping into a face from the past can be unsettling and there are no rules on how to engage with someone you've been on one or more dates with. I'm quite sure that two out of the three boys above didn't see me and stupidly, the one I actively hid from was the one I really hope I cross paths with again.