No, these are definitely project-specific and not part of any add-on.
To give a better understanding: I have five stages to the one project. The modules are hotlinked into the other stages (in progression, i.e. stage 1 doesn't have any other stages referenced, stage 2 only has stage 1 referenced etc) and placed on a master hotlink layer suffixed with a "STGx" where x is the stage number.
I am getting instances where (for example) STG2 is appearing in the stage 1 file, but there is nothing placed on that layer (yes, I've checked all the hotlinks and where they're placed). I've searched all stories, all sections, elevations, worksheet windows...nothing. Deleting the layer seems to have no ill effect, but I can't figure out why it won't purge - and I don't like deleting layers as there just may be something somewhere using it.
To my mind, if I could get a list of the layers and what is placed on them, that would close the loop in the process. But I don't think ArchiCAD can do that yet (but it wouldn't be too hard I imagine as the information is already in the file somewhere).