Detective Xeno and the Seven Locked Murder Rooms - Vol. 2 Ch. 15 - The Train Carrying Death 1

[MangaDex]

 

[Kisato]

I'm guessing steam from the broiler evaporated into some sort of condensation room and collected in a watertight room the victim was in until he drowned, at which point the room drained away.

 

[LeAnimuEyes]

Calm down there, Sherlock. Leave some speculating for the rest of us.

 

[Everfool]

Purely a speculative guess but I'm gonna say that rain(water) is involved.

 

[firedeity24]

I have some theory but I'll just keep it later until everything is revealed.