Skeleton keys?

[Chops]

I have found the first one and have played the required maps with the required characters multiple times and have had zero luck. Is there something I am missing? How can you tell other than when you are informed after the level ends?

[Tellah]

They seem to pop out of random boxes- I've gotten 1 & 3 pretty quickly, but 2 remains elusive to me. I think it may just be luck.
You can only tell if you got one before the end of the level if you manage to see it before you grab it.