Truck losing air pressure while stopped can’t find leak

[Redland1]

Earlier today I was going to get my first load of sand and I drove onto the scale/silo and set my brakes.A minute or two into the loading process the low air light comes on.I finish getting loaded and drive over to the area where we can roll are tarps up and start looking for a leak with the truck off but find nothing.I end up idling up till the tanks are at 90psi and start driving down the rural road leading away from the sand plant.The highest PSI I got was 100.When I made it to the site I got in line behind a bunch of trucks and every time a truck moved forward I released the brakes and the low air light would come on.Just finished unloading and pulled off the site to an empty site down the road and checked everything again with the truck off.Also with the wheels chocked and brakes released.And with the key in the on position and found nothing.What could be causing this?