A bit late to this thread, but I had both rear air suspension ‘units’, aka air bags/air springs/bellows replaced on my S212 E63 with very low mileage (around 12k from new).
It’s one of those faults that many may live with, or may be unaware of. I only used the car infrequently and was aware of it so had them replaced under warranty. No fault codes were stored and the car passed the usual air suspension tests on star but there was definitely a slow leak.
Mine would drop a few mm overnight and a bit more the longer it was left. Just a small leak on the air bags. The valve block and compressor/pipes etc were fine.
Had my trusted local MB independent fit them, simple job once the parts were supplied by the warranty company.
About £1000/pair for the parts and a few hours labour to fit etc.
If you use your car every day you may not even notice it, perhaps only once the leak gets worse. Wasn’t keen to leave it as the compressor ends up working harder and as I had a warranty I preferred the warranty company paying for the work than me later on down the line!