In the level where you go to the storage area, to get the single control rod. In the room just before the room where you actually find it - the one with a big work surface in the middle, with 2 rooms off to the right:

I opened the second room, the one with the monster in, and it chased me, i ran around the big room a bit, then ran into the room the monster started in, and shut the door.

The door never opened again, i tried and tried, i stayed in there so long i ran out of all my oil, and turned completely insane a few times, however the monster never broke down the door, and the door would never open again... I think the monster was just standing outside it, blocking the door from opening, but without attempting to actually come and attack me.

In the end i had to load an earlier save, and snuck around the room the next time.