The text on the card:
"Special Rules: Place a door token on the first map tile's exit. This door leads to level 3. Level 3 cannot be explored from any other door.
Flip When: A player activates Location 13B's equipment(found in level 3.)"
The problem is: Level 3 is a "locked" level. This means you can't enter it until you find the key, which (according to the text on the card) is found BEHIND the locked door of level 3.
However, when you explore Level 2, you'll realize this mission setup card should refer to 12B, which is where the "unlock" card can be found.
Really liking this one so far!