The mask is Bernard from The Rescuers. Wikipedia says "the film is about the Rescue Aid Society, an international mouse organization headquartered in New York City and shadowing the United Nations, dedicated to helping abduction victims around the world at large."
I smell a rat.
4chan looked into this and couldn't find any evidence that the mask had ever been mass produced. If it was, it must be a super rare collectors' item that would not be lying around Spade's house. More likely, it was custom made, and the bad guys made him wear it after Kate's "suicide". The question is why…
We think Disney is putting subliminal shit into their cartoons for the purpose of MK-Ultra mindfucking kids who are ritually abused. There is plenty of autism on this so I won't bother to link it.
Imagine this: kids watch The Rescuers → get raped by satanists → another satanist shows up in Bernard mask to "rescue" them → rapes them even more horribly → personality fragments → Bernard mask is now the "trigger". Since it doesn't exist in the wild, access to the new personality is easily controlled.
IMHO, Andy and/or Kate were somehow involved in a SRA ritual involving a Bernard mask. This is a message to the other cultists who were there. This is their way of showing that it was a cabal hit. Since they like their symbolism, the mask might also have been involved in the killing somehow, i.e. the handler showed up as Bernard to put Kate in the right frame of mind or some sick shit. This last part is just speculation.