This subject pops up pretty regularly here. If I am going to draft behind someone I come across on on a ride, I'll say something like "You mind if I tag along back here for a little bit?* A couple of times I have picked up an unexpected tail, and while I don't think it is polite for someone to ride on my backside without making himself known (if it were a "her" I wouldn't care), I won't get bent out of shape about it.