Is flirting with a stranger naughty?

I'm normally very shy, but on the way home from work, on the train, I caught sight of a rather nice looking guy between the seats, he looked up from his newspaper and caught me watching, I then looked away shyly. After a few seconds I glanced again, he must have felt he was being observed, and looked at me again, I smiled shyly and looked away again. This carried on a few more times, each time he looked more surprised, and I gathered a bit of courage and held the eye contact a bit longer. I had to get off the first stop, he was obviously going further, which I had guessed by the fact he had taken his coat off and sitting comfortably, or I would not have done it out of safety reasons! I came off the train smiling and feeling perky, he had been smiling a lot too, is this naughty, or just harmless fun?