I brought this issue to the antique radio newsgroup since they're
hands-on electronics folks. We batted it around a bit. My
suspicion is that the problem is caused by that nasty little
new-fangled volume control -- the type that's so
automatically-electronic that its last position is stored in
computer memory -- it is not a traditional mechanical control. It
has no "full down" stop, but rotates continuously. All of its
control is performed by reference to the last stored position,
not the control's actual physical position (got that?). I mean,
like, who needs this? It's user-hostile design.

The part is being made by some supplier, and being sold to
various companies. I think that the failure is caused by a tiny
unsecured part inside the control, which has a conventional
"on/off" feel, but in fact, isn't a mechanical switch. Hitting a
bump causes that part to make brief contact, which turns the
radio on. Note that in every case when the radio has misbehaved
in two Hyundais, the failure mode has been that hitting a bump
turns the radio on. When the radio is on, the opposite doesn't
happen: the radio stays on.

Does anyone know of a brand other than Hyundai in which this
happens?

Richard

I haven't seen this issue with the Kenwood.  While the Kenwood isn't the
most user-friendly radio, it does seem to be more reliable than the other
Hyundai radios.  It probably won't happen again if you take it to the
dealer to be fixed.  If they agree there's a problem, they'll replace it
with a reman.