Seems my wife left the Sirius/Audiovox head on after bringing my daughter home from school on Thursday. Got in Friday afternoon (about 28 hours later) and the battery was down far enough that it wouldn't crank the engine. This is a strong battery and a good charging system and there was no other drain that I could find. Fortunately it happened in the driveway.
It may seem like a common sense thing that should go without saying, but I can easily see somebody doing the same thing out on a trip, say, and getting stuck, so there's yer heads up.

Cya,
Robert