>>"q vol HO1302" shows it's in primary tape pool
>>"q libvol 3494 HO1302" said no match found
>>"mtlib -l /dev/lmcp0 -qV -VHO1302" shows

I don't know which level ADSM you'r running, but we had exactly the same
problems with adsm 3.1.2.55 (AIX 4.2.1, Atape 5.0.2.0, atldd 4.0.1.0):
volumes are "not present in library", but still exist in LM and ADSM.

At the same time we had also the opposite problem, suddenly volumes only exist
in the LM and not anymore in ADSM (ADSM askes for CHECKIN).

After upgrading to 3.7.3.x problems were gone.

An other possibility is that you have a bad barcode reader in your LM.

Cheers,
Henk.