It doen't look like a SSB bottle would fit unless it where like a very tiny bottle or maybe a shot glass...how large is this bag? Even then its shape would be visible if the object is in the bag in this pic.
I think it's definately something flat and I'm sticking with my pendant idea yet a comemorative coin or moolah bit would be awesome too!
Or it could be the rest of the mudokon emotes, that's also a possibility