I have gathered from reading that it seems to be related to opening bins from other projects. I don't find myself in the scenario very often, so I don't know about that, however it's showing up in my project in a circumstance where I'm not opening bins from another project.
It seems to have suddenly popped up when I made a subfolder whilst organising my bins in to folders. I dragged a bin from the project root in to this subfolder and suddenly a new folder in italicised text popped up called 'other bins' and there appears to be a duplicate of the bin I just moved in there.
The duplicate bin was unopenable, with no warnings or errors on attempting to open and the project it said it came from was the project currently open and being edited. The entire 'other bins' folder was deletable, though now unrecoverable, but fortunately I don't seem to have needed it. Just wondering what would make Avid think I was opening bins from another project when I wasn't and if there's any implications to that.