There are a couple of bridge sections on my usual ride that make a noise when you ride over them. Often I don't know I have a drafter until I hear a second "bump" behind me after I pass over it. I even had an unknown drafter complain once when I slowed down and he wasn't expecting it.

Maybe there's a market for a sensor that will tell me if there's somebody latched on to my wheel (and maybe triggers a flashing light on my seat that says "back off"!).

For both people's safety the person drafting should make their presence known, and give the lead rider a chance to make their own choice as to whether to participate.