did they need to raise the headroom of whatever area is below it? perhaps to run some plumbing / electrical / HVAC?

or is there any way to access what is underneath, maybe a place for safekeeping?

Theres a full basement so I will have to investigate and see.

Our house once had a trap door to the cellar. It was replaced with stairs in another spot because the area of the trap door was too small for stairs. The result is we have a large, easily accessible basement, allowing a workshop and laundry room.

So, it's possible the reason for your closet's floor is long gone.