Wow, I've never noticed that. I'll have to pay more attention when I change citation detail. I always edit the citation detail after merging a record and I've updated citation detail on a lot of sources as I've learned more about proper documentation. At this point, I have no idea of the exact steps I took with this source citation as it was done back in April and I just noticed this issue within the last two weeks.
I haven't yet tried deleting this "floating" source citation from the online tree because I sent a message to support and I'm hoping someone will actually take a look. I have yet to hear from them. A problem like this would probably go to the bottom of their to do list but the glitch needs to be corrected.