What would you do...

...if you thought you found 150 xtra square feet? We just moved into our new condo and seem to have found a mystery space. All documents say that the space above the 1st floor kitchen/on the other side of a wall on our 2nd floor, is a completely walled-in empty space. Above it is roof deck, one side is a shared stairwell wall, 2 sides are exterior and the 4th is inside our condo on the 2nd floor. Would you take a sledgehammer to the 2nd fl wall to see what's on the other side? Do we have to notify anyone of this, or can we just start making plans to make the space livable? Does anyone have any clue as to why an architect or builder would have done this? We're open to hearing any & all suggestions or comments!