Actually, Nick we think there really is a bug.  I saw something similar once
on Netbackup.  Essentially, the 3494 inventory count got off from the actual
number of entries presented in the SEARCH=YES type CHECKIN equivalent in
NetBackup.  After we ran a full offline inventory of the library the problem
went away for a week or two and would come back.  Eventually, we got a LM
code level that apparently fixed the corruption problem.  Have not seen it
for a long time.