I'll try and elaborate on the latter stories at some point.
Many theatres have butterfly ghost stories - generally inspired by the tendency of butterflies to appear randomly out of season, wakened from their torpor by the heat of lighting rigs. Quite often this happens during shows, and on stage - and it's easy to read the conscious movements into the random flutterings, if you are so inclined. As a schoolboy stagehand, during a performance of the play
Duet for One, I actually once saw a butterfly fly gracefully around the set for several minutes before finally settling on the lead actresses left breast - in exactly the position you might wear a brooch - where it remained for the rest of the act.
The one at Bath is a bit different - it's an actual piece of scenery that hangs permanently from the grid and is never moved. (I should maybe say that I have not been to the Theatre Royal Bath since the early noughties, and maybe things have changed since then.)
It is, as far as I know, decades old and although I don't know the precise backstory attached to it, or indeed if anyone else really does, I do know that act of moving or touching it was always treated with extreme care...dread, even. A friend of mine, who had been a flyman at the theatre in the 80's and early 90's made the mistake of smacking the piece quite severely with another flying bar. He apparently spent his entire lunchbreak on the fly gallery 'apologising' to the butterfly (I'm not sure if this was in some sort of meditative way, or if he actually went up and chatted to it for an hour!) Basically the story was that bad shit happened if you were not careful with the butterfly. Although I asked, I'm pretty sure no-one was really sure of the provenance - but I do know that at the time I knew the place, the subject was treated seriously by the stage crew. There is an official tale that it was some kind of good-luck symbol associated with a change in fortunes after a particularly disaster ridden pantomime production from the 1940's - but my impression was that some people felt there was more to it than that.
There are possibly also stories associated with a small vestibule area that connected the stage to the auditorium, I think on stage left/auditorium right. I don't know how universal this was, but when I worked overnight once with the friend mentioned above (by this time my friend no longer worked at the building, but we were both working in installation work for major shows) he absolutely refused to pass through the room. That said, we were working alone in the building and did love to shit each other up with spooky stories.
A couple of warnings regarding theatre ghosts:
Theatre managements - at least in the UK -
love them, and will be on the least of stories for a bit of publicity like a rat up a pipe. This has a tendency to turn one or two particular tales into what becomes, in effect, the official line, tends to change or solidify them in the process of making them palatable to the readers of websites and promotional literature, and often has little to do with anything that might actually be being experienced within the building.
Secondly. You'll hear many stories from actors, because, of course, we automatically associate actors with theatres. However actors spend a fraction of the time inside those buildings that technicians do. If you really want to know what's going on in a theatre, you're better off talking to the stage crew or the stage doorman (who, at least in the old days, would know
everything), not the turns.