This is the "Missing Mighty" poster.
And....it's a little bit overly simple? The idea behind it is fairly cute, a 'wanted' type or 'missing children' type post-up poster, but for Mighty the Armadillo who had not been seen since Knuckles Chaotix. A careful eye would spot several of these (for Mighty & Ray) in other Sonic games as little background nods.
This poster attempts to bring the 'easter-egg' background into real life, with printed on 'stains' and 'tattered' edges like a post up poster. However, the poster itself is a large gray rectangle with the faux edges printed on. It says "Mighty the Armadillo: Have You Seen Him?" But that's it, there's no 'call 555-SONIC' or anything to complete the look of a real poster. Then, it uses the simplified cute-style art from the Mania Youtube Cartoon for Mighty himself. This poster is big too, so it has plenty of space for the design to seem a tad plain. It is 16 dollars in the SegaShop online in 2018