I brought my e53 back from Munich on a trailer (on Nov 14 right after the Paris attacks
) behind our 9-seat tow vehicle. I had lashed the wheels down tight to the trailer, so the x5 suspension was free to cycle as we drove.
Arriving home, I found the battery dead. Odd. I sussed that out the other day whilst putting on the new number plates ... I heard the air suspension compressor start up. The car key was in my pocket at the time.
So all the way home from Munich on the trailer, the compressor must have started up from time to time and depleted the battery. The battery is also weak and slated for replacement tomorrow.
I would think that the compressor would/should be on a switched power circuit. Is that silly of me? Are your cars similarly wired?