[Teutone]

Locked keys in W126 trunks have been known to elicit spontaneous expressions of religious devotion.....

To have the extra trunk latch might make matters worse

 

[Teutone]

A locked W126 trunk is not easy to get into, especially if it has the second latch.
Breaking through a tail light, or reaching through the openings under the rear deck trays works to an extend.
BTW, if you turn the lock to the right and remove the key from the far right position, the trunk will stay locked even when other doors are later opened, or the car is started.