x5 rear air suspension

[venom290]

Good morning all.
2001 x5 4.4 rear air suspension only . Randomly started to lose air overnight on left rear . Sometimes not for a week . Sometimes 2 days in a row.
Compressor comes on as needed, runs fine. Checked all lines, connections, both tanks, and both bags as best as I could with soapy water. No leaking that i could find .
Where it doesn't happen a lot , or consistently, i think I have ruled out a leak.
Possibly a sensor? Any ideas? Way to test ?
Thanks in advance, jeff