I get these every now and then on the recumbent. It's usually other cyclists who're surprised by the speed differential and the lowness of my recumbent, despite me giving them loads of space. Frankly, I think it's because I'm on another quiet bicycle, and they're not looking back because they're wrongly relying on their hearing.
However, I had two awesome ones on my Thursday commute. Both car drivers, stationary, as I passed them to enter the ASL. LOLOL!!!
