People do this, and yes, my reaction is usually,
"what the fuck?" I got used to this far too
often, people laughing, and smiling, when I
was hitching, yet driving on.
But, even outside of this experience, it
leaves one wondering who the fuck that
was. Or startling you, thinking that they're
trying to warn you about something.
I put it about on par with when an acquaintance
of mine would lean out the window and shout
to people on bikes - "your wheels are turning"