It's well established the great visual gags that Muppetvision 3D has all around it. One such gag is when you first enter the building on the right side. There, you'll find the security office. Whomever is supposed to be in charge has abandoned his or her post and left this sign in the meantime.
This alone is a nice little joke, insinuating that Muppet security is so lax as to leave the key to the building underneath a floormat. What really makes this entertaining is there really is a key under the floormat. If you backtrack a few steps (and ignore the looks other guests give you as you attempt to do this), you'll find a key to the building underneath the floormat.