also, the single-field edit didn't remove the item - here it's flagged as 'not in library.' Oddly, pressing 'ok' to save doesn't remove it from list as I would have expected from post #2 and
previously seen behaviour)
Since the category editor was mentioned, I still think the most likely situation is it happened when I was editing items in #kobocoll (merging and renaming) which would've updated the user-category. Though [Send to Device] is in #kobostatus, so I'm not sure.