The cabinet is on the floor ground of the hospital, not underground, right next to the entrance to the cave

Here

That's the cabinet it opens, it took me too long to go back up the hospital, I was running round and round the underground.

theres no box to use the key tho . . . .

You need to finish that quest act before the key will spawn. So make sure you're at Chapter 3 act 5 before looking.

 Click to Enlarge

You need to finish that quest chapter before the key will spawn. So make sure you're at Chapter 3 act 5 before looking.

 Click to Enlarge

The key isn't there for me either, this article may need an update.

I would like to see an update, I just checked the key's description and it says it was stolen by a weasel, my bad was not hitting the mf'er when I saw it come out of that sand pile, so now I'm waiting for it to respawn

 Click to Enlarge

Guess my account's glitched, cos the key most definitely isn't there ¯\_(ツ)_/¯