Given we have a room system, I think that perhaps the Dementor attacks are room-independent.
However, AK is a short-range attack, so I'm guessing you might have to be in the same room as the attacker. So Ruby, what room were you in last night? Not that we know what room anyone was in, so it won't really help, but still... I'm guessing that the AK-users are a team, and that as long as one of the team is in the same room as someone on their list, that person will be attacked. (at least I think that's how the Wiki said that type of thing would work)
I could see poison being either a room-specific attack (opportunity) or a long-range attack (dill pickles, anyone?).
I'm guessing that Rachel's disappearance was a room effect. I chose the owlery last night, so I would guess she did too, and that's why I was the one who saw the large bird snatch her.
And yes, I realize Ruby was attacked near the owlery, so she might have been in that room; just to set the record straight while I'm away at work, I did NOT attack her. I wonder if it could have been Rachel, or someone else who was in the room and remained secret. On average, there were 4-5 people in each room last night, so there were probably 1-2 others unknown in the owlery.
Still, it's the only clue I have to go on, so I'll
Vote: Rachel(I'll check after work and change my vote if more experienced players think my logic is completely flawed, or if Ruby was not in the owlery, after all. And the note said Rachel could not vote, but it didn't say she couldn't be voted for, right?)