Most Helpful Guy
I hope that you don't take this the wrong way, but maybe he's just now starting to notice you. There was a girl in one of my classes that had a huge crush on me, and I became embarrassed later when I realized how obvious it was. So, at first, I only saw her as a friend, but, over time, I began noticing how much she liked me, and, because people like those who like them (or appear to like them), my feelings slowly started changing. I wound up developing a crush on her, but, since it was so long after we initially meet and she had kind of cold off, I was worried that she may have started to look at me as a friend, so I was unsure how to act. There were other factors to, but it'd take a while to explain the entire thing so I'll skip it. Moral of the story, it's truly amazing how oblivious guys (or, at least, I) can be when it comes to noticing signs, even really, really big and obvious ones.
Back to your situation, it was only one time, and there could have been many legitimate reasons for which he had to take off on that day. And, really, even if he starts doing it regularly, he could be doing it because his work schedule may have changed or something similar. Just try and show him interest and have fun around him, and he'll probably figure it out sooner or later.