Tiny Key is an item in House Party. Ashley keeps it hidden in her top. It can be used to unlock the Bike Lock on the closet in the Spare Room, to gain access to Ashley's snake Julius Squeezer.
If you untie Ashley's top, the Tiny Key will fall out. Katherine will find it and approach you later to give it to you.
If you wash and dry Ashley's Clothes during A Change of Heart or Humiliate Ashley, you will find the Tiny Key while taking the clothes out of the dryer.
If you haven't untied Ashley's top or cleaned her clothes, then you can get the Tiny Key by having sex with her at the end of Smooth Operator - Ashley or Sibling Warfare. Following the cutscene, the Tiny Key will be left on the desk in the Spare Room.