The Mysterious Baseplate is a baseplate which sits outside the map, unnoticed. Below is a picture of what it looks like. In the picture, you can see a smooth, grey baseplate with 2 diamonds and 1 text orbs, with the diamonds being outside of the baseplate.
"I have a theory, a theory that I think this baseplate might just be the new map, a model for a new type of texting tool, or an obby. I guess it would most likely be a map, since models don't be this big, and not a obby either, because of the same reason for the model, but the emoji orbs and diamonds don't appear by the model, you get some from their rewards when you complete them."
-STOPTAKINGALLTHEUSERNAMESeeeeeeeee, the one who created this page.
How To Get To It
First, get the tape from the droid quest, and then do the parkour. You should be able to see it, along with a Unknown Door, and the rooms of the obbies.