Haha, recently bought a new home which also had a safe embedded in the concrete under the flooring.Place was apparently owned by a dude who had been in jail, a few owners back, to add to the 'story'.
Key was present but not the code, which would be useless as the know was rusted in place and wouldn't move.
In the end we drilled a hole through the top plate (did not take very long but made a lot of noise) and stuck a little camera through to find it was empty (as far as we could see).
Cemented over it, new floor over that, left as a surprise to future owners.
It would've been fun to put some piece of treasure for the next adventurer. Maybe a time capsule or even a single silver coin (only about $20) would really delight someone. This has me thinking I should do something like this in my house.
In the end we drilled a hole through the top plate (did not take very long but made a lot of noise) and stuck a little camera through to find it was empty (as far as we could see).
Cemented over it, new floor over that, left as a surprise to future owners.