There always have to be holes to allow positive air pressure out - notably from the aircon. You probably own an airconditioning unit at home where you have to make sure the windows are open or something, too.
Then you've got small leaks around drain holes, door seals, etc.
Proud TQE Operator and President of SEFAG - the Society for the Elimination of Ford Automatic Gearboxes
Pacemaker headers + 2.5" exhaust
EF Fairmont interior
Lowered King springs
5 speed manual
Newest mod: EF Ghia wiring loom and dash