Always ask who keeps the key
The most reliable way to understand a closed room is to find the person responsible for opening it.
Every locked door has a keeper, and the keeper is almost never the owner. It is a caretaker, a neighbour, a retired employee who was never formally relieved of the duty and simply kept doing it.
Keepers talk, if you ask about the routine rather than the mystery. How often do you go in. What do you check. Has that always been the arrangement. The answers assemble a history no plaque will give you.
It is also the honest way to do this work. A room with a keeper is not abandoned, whatever it looks like from outside, and turning up uninvited is trespass dressed as curiosity.