There are still two likely routes (Moe's, left by previous occupants). An ignorant Chii is still the comic dark horse.
Nico's throwaway "I lend it to friends once in a while" (friends, plural? They fuckin') is quite conspicuous, but it'd be a pretty wild coincidence for the offending item to just happen to lie on the ground a few inches away from Moe's opened suitcase when Ichikawa found it. Then again, it could also have fallen off the cabinet somehow while Ichikawa was looking around for hidden cameras.
I don't really want to assume it was lying there in plain sight all along, when it's in open view of the entrance. Just seems unlikely nobody would spot it when they enter the room for the first time. It was kept tidy and there was nothing else on the floor.
Moe's ominous "You can't flirt around too much" towards Ichikawa suggests to me she believes it's absolutely a possibility the two of them might bone down. (Way back when, chapter 50-something, she thought they already did.) Her plan could have been to get Yamada alone at some point, to tell her: "It's dangerous to go alone, take this."
Kobayashi is exceptionally unlikely simply because she didn't even unpack in the bedroom - she picked the sofa in the living room.