I'm having a similar issue with my SLS. In particular, the left side.

The left rear airbag is sometimes sagging when the engine is off, the right side is unaffected. It first happened about two weeks ago. I started it up, closed the doors and it came back up and stayed that way. Over the past five days the the drop has occurred more often, although it seems to be random - sometimes it'll drop as I turn her off; other times it will stay up thru the night.

When it does drop, I can hear air being released. Given that it is isolated to the left side of the car I am leaning toward an issue with the solenoid valves located in the compressor housing. Does that make sense? Or could it be a problem with the ECU? Could the level sensor be compromised and be providing incorrect data?

Lastly, does anyone have an idea as to the cost to replace any of the above units?

Thanks in advance for the help!

Be well,

Jim