Well, discord’s down for me, so I’m just gonna throw out ideas here.
Assuming the culprit isn’t the butler, the culprit would’ve had no guarantee when it came to the butler and maid’s movements. If something in the room had made the butler sick, there’s nothing to explain why the maid wouldn’t have gotten sick, as well. In other words, there was no way to predict them going to the bathroom, pretty much eliminating most, if not all, sort of hiding tricks.
The only thing that would escape this would be something like the person that the two had seen was the killer pretending to be the victim (somehow?) and that, while they were in the bathroom, the killer brought out the real body (from somewhere outside of the room, presumably?) This, however, relies on the fact that the two would’ve gone into the bathroom which, as I’ve said, there was no real guarantee of doing.
So, if we assume no hiding tricks, it would mean the killer would’ve had to have somehow left the room by setting the chain from the outside - since locking the windows from the outside after escaping is more or less ruled out.
So, chain’s the best bet for now. Hasn’t been denied it had to have been set by a human hand still, so: the killer used a string running from underneath the door, going around the spear for the sake of gaining height, and using it to set the chain by having the string pulled, setting it in place.
Thinking about it, I seem to remember you once doing a string trick that would’ve locked a door from the inside… I just can’t remember if it set the chain, though.