There are actually two instances of it. The easiest one to find is by completing the maze game at the back left of the mansion. That will open the door with the picture of a tiger girl done by bubbles pup.
22 days ago(+1)
Would it require you to find the common key to open that door? Unless it is hidden somewhere obvious and I'm just incredibly blind, but I don't exactly know where the common key is hidden.
Sorry if I am asking a lot of questions, but I am not sure if I am missing something obvious during my playthrough
22 days ago (+2)
Seek out the bunny. 
I was a bit too evil with this one as it's VERY easy to overlook even with the ginny note stating a key was hidden in this room. I might move it and I am going to give offer a 2nd placement for the key since it's far too early of a stop gap for players.
22 days ago(+1)
You aren't the first. There is very poor lighting there and you can easily miss it even with the cursor hint. I am going to add a sparkle effect to the keys like the lantern has.