Look at the check valves on your front and rear tanks.
Sometimes shaving cream in place of soapy water will “show” smaller leaks.
Lots of valves will leak internally and make it hard to find.
Cummins ISX check the turbo actuator control valve... sliver box, drivers side below the air compressor. Follow braided line over to turbo and check for leaks.
Engine fan hub and associated lines and connections.
Parking brake valves and lines are bad apples for leaking.
Above the air tanks located on the inboard side of the frame rail is a “brake control valve”. Looks like a junction box of lines. Gen 1 Volvo’s have 6 lines going in and out and Gen 2 have 8. Leaks internally all the time. Some airtank fittings have a protective boot to keep out crud. These sometimes will mask a leak.
Sometimes you need to “wiggle” the air lines a bit to hear the leaks.
Temperature changes will also reveal new leaks.
If you do find a PTC fitting leaking you can usually repair it by replacing the o-ring inside instead of buying a new fitting.
As mentioned air bags and leveling valves can leak and drive you crazy! You can inflate your bags to “reveal” the bottom edge and check for weather cracking. Lots of bags look ok but need replacing.
If you can “isolate” your front and rear tanks you can get a better picture of what may be causing your leaks. Front system includes a lot of your accessories, such as seats. Rear is mainly rear brake system and such.
Do you have an air ride hitch? Check it out.
Just thinking out load....