Air is vented into the tank via the vented fuel cap. Air doing what air does, enters to fill the area that gas once occupied. It also aides in the fuel siphoning to the fuel pump.
Since fuel is heavier than air there should be no air getting below the fuel in the tank if it's a bottom feed tank. If it's a top feed sender/fuel pickup air can be introduced at several points in the pickup tube. Since the fuel pickup/sender is encased in the tank there wouldn't be any evidence of a leak.
I don't know if his tank is in the cab on a 3600 or if it has a top feed/sender but if it is and does, that could be a possibility.
