There would have to be enough open space for the seeds to blow in to the room and enough moisture in the carpet so the seeds would want to germinate. I would guess that there are rotting wood floors below the carpet so that the plants would have enough depth to take root.

Don't try this at home ;)

This looks like a TB ward. Where the patients were able to get outside air.

I had one of these that came out of a storage room in a fire department. I found that they are not particularly valuable. I got $25.00 for mine and it was in good, working condition.