The sales record must be corrupted?
Check your SOP10201 table,
SELECT SERLTNUM
,POSTED
FROM sop10201
WHERE QTYTYPE = 1
AND ITEMNMBR = '{your item}'
For the sales order line combination that the item was sold on, does it have a corresponding line for the serial number, and is it marked as POSTED.
I suspect one of those two will not be the case.
Thus inventory rec is simply putting it back into stock as I can't see a sale for that serial.
Don't play with the database unless you are confident and have backups, but you could fix it there...
Tim.