Once a door is opened, it is considered to be an empty space. I think some terminology may be required in the rule book to distinguish between unoccupied (i.e. no figure in the hex) and empty (i.e. no figure, obstacle, trap, closed door, etc.). The proximity mine should probably then read "empty," so you wouldn't be able to place it on top of another trap.
Specific wording is very important to avoid rules confusions. Glad things like this are getting worked out before it's sent to the printers.