First, I'm not one of the guys who'd driving around with the air ride system so take this with a grain of salt.
The air system fills the chamber in the front shocks above the oil so even though you checked the fittings leading to the front shocks and the schrader valve, did you soap up the rubber boot (dust cover) on the top of the lower half of the shock to check for a leaking seal? And while I'm talking about this, it would be easier to slide the dust cover up the inner shock tube and soap up the oil seal itself. Chances are the "oil seal" is leaking.
As for the rears, the air shocks are similar to the front shocks in design. The rubber boot over the outer is a dust boot I believe. You should be able to pull it off of the outer shock housing and do a leak check with your soapy water at the shock stem.
Not sure what you're using for "soapy water". My choice for soapy water is a 1/2 litre of water and 5cc's or 1/2 teaspoon of dish soap. If you can't "blow bubbles" with your mixture likes the kids do in the bath tub, then your mix isn't good enough to do your testing with and you won't "see" the leak.
