if the tape contained a TSM database backup (q volhist type=dbb) then your
setting for drmdbbackupexpiredays has come into affect. find out the setting
for that with 'q drmstatus'

if the tape contained normal client data, then your setting for noreusedelay
will keep the tape offsite until it's passed. this setting shold be greater
than or equal to your drmdbbackupexpiredays setting. find noreusedelay in
your storage pool attributes (delay period for volume reuse)

these two reasons COULD explain why the tape wasn't returned.

hope this helps..

Matthew