I'm no expert but from what I have read just running the engine without driving exacerbates engine wear. This ties in with what is normally written in the BMW owners manual, it usually states that you should not sit with the car idling, it is better to get the car up to temperature by driving the car.
I would imagine that the oil pump & water pump are not working at full efficiency when idling.
I have my car on axle stands to prevent flat spots developing on the tyres. I also have moisture traps in the cabin & trunk. I used to run a dehumidifier in the car regularly too. I made sure that the underside was as clean & dry as possible before tucking it away. If you're taking it on short drives is the car always clean & dry going back in to the garage?
Maybe I'm over thinking it...