I had a similar situation last year and had convinced myself it wasn't the air bag as you could never catch it in the act of leaking. It only happened now and again and seemed to depend on how you had parked the vehicle. One day it was persisting down with rain and I heard the leak and saw the bubbles with all the moisture. Rust on the air bag seat had gradually worn a pin-hole in the bag but as stage recovery suggested, the fold in the air-bag often sealed the leak until you happened to park with that wheel in a dip. Suspension would automatically level and inflate the bag and straighten the fold then it would leak. Change the bag, it is very easy and not that expensive.
Calum.