I know this post is 5 years old, but I just encountered this myself. I looked around in GECK for the vanilla game and could not find it in the cave. So then since I found information on Loyal's computer about it being a caravan deck it hit me that this is clearly from a card mod. So I read the readme for Rapscallion's Sexxxy Card Revamp 3-0 and sure enough it is mentioned there. The key IS there, but only after you've recovered the lady of the lake. If you're having trouble check out the mods readme! Take care!
