I've had that happen on the Sammamish river trail. Well, not exactly that, but I'll look back and some ghosty guy has crept up behind me and is cruising along just behind my back wheel--drafting, I guess. It's not like I'm any speed demon, so it seems super creepy (I'm usually alone). What's he doing THAT for? I usually slow down to the point it becomes clear that I don't want him there, and then he speeds off and disappears in the distance. There's another guy on the same trail who seems to be the bicycle police. I've caught him skulking behind me at least twice and each time he's sped up alongside to aprise me of some issue my bike is having that I was well aware of before he too speeds off and disappears in the distance. Maybe there are just a lot of well meaning guys out there who are unaware of the concept of "personal space" and that it might also extend beyond one's bike. Or maybe they really are creepy. I donno.