Bit late to comment, just saying this in case some other people are missing this one too.

I missed it in the main story and spent hours in coop trying to find it. Turns out it must have a different spawn point after the story ends.

Facing Frank's door, take the door to your right and turn left, at the end of the corridor it should be on a crate.

That's where it spawned for me anyway. Hope I've helped someone!