I know it is unusual, but sometimes there are tags in the processor that are not used in logic, but are used by an HMI or some other PC based system that will not show up as unused. After deleting you will then have some issues with the other systems involved. Like I said this is unusual and most often happens on old systems that multiple people have been involved in.
Anyway, if you do this online it will not let you delete any tags that are being communicated with and if I remember right it will give you a message telling you it could not delete a particular tag. This can be helpful in figuring out what un-necessary communication that may be going on that can also be removed from the HMIs or other software.
Darren