Gas flap: passenger side behind the trunk hinge. Pull out the plastic trunk liner and you can't miss it.

Under the hood: Hook up a hand vaccum pump to the yellow (those are door lock) lines, check to see if they hold vacuum.

Doors: Inside at the bottom (held in with two 8mm bolts) is the locking element. Driver's side is different, it just has a valve. Check all rubber fittings for cracks.