Don't you hate it when someone responds to their own post?

Anyway, today I had this problem, and managed to get the slots
back.  For TODAYs answer (other things could cause it, so the
answer might be different for you):

        q libv

I copied the listing from the 'q libv' into a spread sheet (anywhere should
be ok, this was convenient for me).

        checkin libv libraryname search=y checkin=pri checkl=b

Scan the library for volumes that are not 'in the library', and make the
private.  Once it is done,

        q libv

I copied the listing into some more columns in the spreadsheet, and found
the two 'missing' tapes that really were in the library.

I did a 'q vol volumename f=d' for each tape, determined its status, and
updated it as was appropriate for the specific tape.

I hope this helps someone.