According to the schematics in 36-11-11 and 49-52-31 the ground to turn on the light comes from the Engine bleed switches and the APU Bleed Control Valve. If the APU Bleed switch was off and you still had a Dual Bleed light you had a malfunction of the Bleed Valve in the APU compartment or a short somewhere between the APU and the cockpit.
At least that's what the schematics for my company show.
