Stereos can be pretty bad for this if they are not wired properly. Especially when there is a seperate amp involved. Amps and HU's (assuming aftermarket) have two power inputs - constant & switched. If these are mixed, the wrong parts can be kept alive and drain the battery. Even if just the amp is wired wrong, it can reverse power the HU for a double wammy.
Also remember (well in my 325 anyway) that there is no switched power to the HU. It's live all the time.