Originally Posted by My Gamertag is Papa Skull
The most common pass key that people miss (including me) was the one in the water by the Scuba Mask. It doesn't show a question mark because there can only be one question mark per square so it didn't show a question mark because you already grabbed the Scuba Mask.
Go check and see what happens, good luck.
Wow ... I spent SO many hours looking for the "last" passkey, and this was where it also was for me (I still have the true last one that's at the bridge on the top left of the map you have to use hyperspeed to get. Serious PITA for me.).