Chryse,
You are missing my point.Even with the APU off,you can still get a back pressure causing the engine bleed air to drive the APU.Admittedly you would need a dual failure,but if the check valve was stuck in the open position AND the APU bleed valve was also stuck open,there is nothing to stop the engine bleed air from driving the APU via the compressor blades.That is what I meant by saying it's irrelevant if the APU is running or not.It could be argued that having the APU off is worse than having it running in this case.Remember,this is only a problem in the unlikely case of both check valve and APU bleed valve being stuck open.
By the way,there is no indication as to the position of the check valve,it's a simple flapper valve and the dual bleed light receives an earth through the 'open' switch in the bleed valve itself and the engine bleed switches as is stated above.