As set of test Categories were made, nothing was done with them, no items added.
These categories were then deleted, but they are still selectable within Add/Edit Items and do not appear in the category list.

It's like at home - you have to bring your trash in front of the door not only in the trashcan in the kitchen ;)
There is an Field for Cat and one for Articel Trash.
Empty the K2 Trashcan :)

I see there is Trashed Items and Trashed Categories, that is no problem.
If an item is trashed, you can't see it or use it, unless you go the trash.
If a category is trashed, you can't see it unless you go to trash but you can use it, which does not seem right to me.

If it is in the trash, you put it there as you don't want to use it, it is just there in case you want to get it back.