>indeed they are checked in as scratch, so there seems to be a bug somewhere
>for the volumes not to automattically return to scratch

Again, check your Activity Log for what events surrounded the historical use
of the volumes.  (Your Activity Log retention, one way or another, should extend
back in time at least several months, as necessary evidence of what's happening
in your company's backup system.)  If the volumes were Defined to be in a 
storage
pool, they would stay in the storage pool when emptied.