Rear end sag, is this normal?

[wsybert]

I tried searching with no luck. Mostly found issues with the E53 air suspension.

Vehicle is an '08 E70 4.8i.
I come out this morning and the vehicle looks low at the rear. Low enough that it looked odd. So I grab a tape measure and the left rear is 1.5 inches lower than the right side. Hmmmmm... So I get in and start it up and let it idle for a minute or two. Shut it off and check it. It's better but still almost an inch out.

So I jump in my company vehicle and drive to work. (the X5 sits for 3-5 days at a time.) I get home jump in the X and take my son somewhere. Get back and think to measure it again. Both sides are equal.

I can't believe this is normal. Any similar issues or comments?
I'll be checking it in the morning, but I am guessing the air suspension leaked down after sitting for 3 days, then recovered after a 30 min drive.
Thanks
Bill