You know, I've thought about that. I think if you live with others though, a note outside the door is going to alert them & make them try and break the door down and possibly interrupt a ctb attempt. Whereas perhaps if the outside of your door looked the same, no note, nothing suspicious, & then perhaps barricade the door the buy time.
I read about a couple that ctb'ed in a hotel room and put up a curtain with a sheet by the entrance of the door (inside), and on it, wrote a note for the maid warning her not to go back that bc they were dead. I was thinking something along those lines might be good. I too am trying to think of some way of warning my father before he sees my body in my apt—maybe a sign that goes across in the middle of the hallway (in my actual apt) that leads into the main room where I'll be. Shit. But a sheet blocks the view in case someone is really being absent-minded and not realizing what looking right away is really going to mean. It gives them a second to think.
That might be hard in a bedroom, but maybe even if you pin the sheet to one wall and then drape it over a piece of taller furniture and get behind it (note on sheet)? I don't know your method. Maybe that won't work at all. I would try to devise a way to have the warning come from inside the room.