My girlfriend drew a card during the game that looks almost just like the rotating room card but it appears to be a staircase. I could not find any explanation for this card in the instruction booklet. Does it serve some purpose? Can you descend down it into the catacombs?

Catacombs. When in doubt look at color of arrow.

So she could have simply decided to descend down those stairs into the catacombs, it seems.

That is correct! She could simply go to the catacombs or (if my memory serves me right) she could also search the room becuase of the torch symbol.