Where have we seen a hat like the one Maxim is wearing before, I wonder?
"Maxim's Exciting Hat Story!" is a one-page "Holiday Hilarity Break" that appears on the website at the end of Volume 9 ✣ . It is also presented at the end of the print edition of Volume 9, Agatha Heterodyne and the Heirs of the Storm.
It takes place in Mamma Gkika's. Maxim tries to tell Mamma, Oggie, and Dimo the "amazink" story of how he acquired an oddly familar-looking hat from a "beeg fat guy", but no one is interested in hearing it.