[CSRoad]

My cap with the right key needs to be pushed down in the tank opening for it to snap back and allow the key to be removed.
Therefore I think @Husoi 's suggestion to remove the rubber seal and taking a look at the mechanism, may hold the key. (Sorry for the bad pun at this difficult time, I couldn't resist.)

 

[CSRoad]

If the locking tongues are retracted that will mean the key has either been inserted in open position which I find odd or the key has been turned and is now locked inside.
Odd because the key can't be inserted or removed if the cap is open to prevent leaving the cap open.

A good point it becomes a logic problem given that from all appearances the key and cap are behaving normally.