What do I do with the key I found

Hint 1 of 7:

"The documents in safe place; here is the key (and paper with 2475)"

Hint 2 of 7:

The number is Not a hotel room.

Hint 3 of 7:

It is in another safety location.

Hint 4 of 7:

It is a safety deposit box at the bank

Hint 5 of 7:

Use Card and key on Bank Clerk in the change cage.