Damn, what a good hypotheses
Viewing post in Version 0.11 - Rune & Devon update comments
I'm just suspicious by nature. Mikko's hug is the one constant event regardless of which table you sit at. Combine that with the fact that Mikko wanted to share a room with the protagonist, as well as the unresolved tension from when the protagonist suddenly grew distant in their past, and you get a plan to try to force the protagonist into sharing a room with Mikko. This would not only bring the two closer together, but also guarantee a time and place for Mikko to confront the protagonist about his past evasiveness.
This theory is actually vastly belived in the discord server, but it obv never got a confirm by the authors.
The only problem i see is the fact you do in fact find the Key even in rune's route, Wich can mean only two things: either Mikko found a way to put it back there overnight or before arvo got in the room or mikko didn't actually took the key.
Another moment where he could have put the Key in the bag could be after Mikko plays the piano, but the bag won't be left alone if you refuse to play too, so that would make little sense.
The only certain things is this is kinda suspicious