These two gorillas can be found on Queen Square, though you have to hunt around for them.
Going Gone is covered in special heat sensitive paint which when warmed up reveals messages beneath and has some special smartphone readable barcode thingy (lost on me I’m afraid).
I think it probably works best from publicity stuff I have read about it if you throw hot water over it. Have a look at this article with photograph of the artist demonstrating how it works. The hot water reveals messages about the plight of the gorilla. It’s a very clever idea but a bit lost on passersby who don’t know the story and have no hot water to hand!