Many years ago, the safe in in the supermarket I worked in developed a fault, so we called in an engineer and I had to chaperone him for several hours whilst he worked.
One of the things I learnt was that inside the door was a large piece of plate glass, which had spring loaded locking pegs attached to it. All the locking mechanisms were behind this glass, and if someone tried to drill through the door the glass would shatter and dead-lock the door. Other things that would have the same effect:
– Explosives
– Trying to move the safe without “disarming” the springs
– Slamming the safe door shut
This was in the early 90s, and the safe was a lot more modern then the OP’s, but I imagine a botched attempt at trying to get it open may make it even more difficult for a “professional”.