Don't forget that the air conditioning system will be exchanging a certain % of stale air with external air (typically 10-20% per hour) when on the ground as well as in flight. You are most likely smelling other aircraft's burnt Jet-A1 as you taxi along/wait in line for T/O.
There will be negligible leakage through doors etc. and as another posted stated, that air would egress from the plane...

- Tim