Honestly. I have the infamous unkown keys (Or at least one or two. I dunno if I have the red one) But like others, I can't find the doors for them.
However, though I have keys with no locks, I've also noticed some locks with no keys. One is the old man you have to scare in Earthrealm's hut, and the other is a rather large building in the netherealm city. I'm also sure there must be more, that I just haven't noticed.
Does anyone know how to open these? Maybe before we go looking for places these keys can get us in, we should concentrate on the places we know we can't get in already.
Just a thought.