Air leak question

Get you a spray bottle of soapy water and spray connections until you see bubbles while it’s cold. Maybe one of your quick connect fittings.

 

What did the mechanic do? Waive a divining rod over the tanks and didn't get any movement?

I would think they would take Banker's advice and use a spray bottle on the most common connections.

 

If it was snow/slush covered when he "looked it over", pretty good chance the walk around could not duplicate issue.
Meaning not a chance in hell he was crawling underneath it to check fittings/connections.
I had same issue couple of yrs ago & found out it was my seat air line fitting.
Couldn't hear it but sure enough slow leak to drain out every night.

 

How much is it draining and over what time period? If it only happens in cold weather its probably a quick connect fitting somewhere. If its only the secondary tank, check your airbag fittings, air ride seat fittings, anything that runs off the secondary tank.. soap and water. Id bet money its a bad quick connect

 

Probably first pulled it in to the shop and let it warm up a bit before getting to work on it.
Was that the mistake?

 

You sure you’re not hearing the air dryer? Mine freezes up and sounds like a leak but it’s not. Just the cartridge frozen.

 

The one way check valve may be iced up as well. See if they can pull the line off the compressor and pour air brake antifreeze in there.