Far from my expertise, but there should be no connection between the combustion side and the conditioned air side. The only thing I can think of would be duct side leakage inside the cabinet then accessing the combustion side through the air intake. Of course there is a possibility of a cracked air exchanger, but not something you would run into on every unit, I would hope. I'll be watching to see what the pros have to say.
Damien, if you do that, your results are inaccurate and of course the HVAC guy has an issue to find & fix
The duct test doesn't test just the ducts, but the whole air handling apparatus. The only exception is the version where the air handler hasn't been installed & then they are only allowed 4% leakage per the 2009 codes. With that said many municipalities don't accept that test & we simply won't perform it