Skintman Quite like you I do understand TS's position and in my practice excercise accordingly. This is my view that attempts to avoid the twilight of security protocols and answer your Q:
Entrance doors on large passanger aircraft do not have any locks. Just a handle to open. As far as security evaluation goes, the reasoning is that once you are allowed airside, and know how to operate an aircraft, you can do harm. Fitting a lock will not stop you and so there is no need to install one.
RE: cockpit door (not a part of your question). In the post 9/11 era, aircraft are equipped with intrusion proof flightdeck doors that are locked. The procedure to open it is not based on a hardware item as this woud be easily obtainable by a would-be criminal. The procedures are far more advanced, airline specific, and must not be discussed on an open forum like this one.
Apart fromsome small aircraft, there is no "key" to start, car-wise. Just a number of pushbuttons, levers, switches, and selectors. The logic suggested above applies.
Yours,
FD (the un-real)