(My son's car). Battery suddenly started going dead after just a few hours
when car is parked. We figured it was the old battery so we bought a new
one. New battery is doing the same thing. We can't find anything that is
"on" when the car is parked.

Any suggestions on what might be causing this? Obviously there's a power
drain somewhere but we can't find anything causing this, or why it suddenly
started doing this. Alternator is OK, and new battery is OK too (both
tested at shop). Any advice appreciated! Thanks.....