P.S. If the APU is not running, a properly functioning APU check valve should stop engine air getting into the part of the APU duct which runs through the pressurised fuselage area. I assume the APU is only used in flight for special circumstances, so the additional seriousness would only be a problem on the ground or when the APU is run in the air.
I have heard of an APU duct leak on a 767 where the passengers in the aft zone on the left hand side started to get a bit warm ... but there was no cargo fire alarm, perhaps because the duct is in the cargo sidewall and the sidewall panels are taped up for additional fire protection. By this, I mean the tape provides an air seal. E.g. if there was a fire inside your house, you would close all the doors to slow down the progress of the fire (less oxygen, less smoke, etc).