I don't work in the airline industry, but using logical process, sim experience and a little guestimation and given 5minutes alone with a Q400 or any other aircraft of choice, I could very probably get it to start and move and taxi and fly, just don't ask me to tune the radios and make accurate radio transmissions. I've no idea what "with Mike or With Zulu" means. A bold claim I know, but I'd prove it cpit opportunity given, and I'd bet a few others in Aces High could do just the same.
There is the absolute possibility that the guy used his companies lax security around him "getting the trash bag" and sim experience to go off and do what he did.
No conspiracies, just an opportunity and (depending on perspective) poor judgement.