OID type creates files that don't go away

I've created several blobs using the OID type.  I note that the database
stores the "oid" number that represents two files on disk, such as xinv21281
and xinx21281 with the oid in the database showing up as 21281 via a SELECT.

But, I deleted a row that contained this oid, and the two 'x' files are
still on disk.  I then ran a VACUUM, saw that 1 record was reaped, but the
two 'x' files remain.  Are these 'x' files going to be reused in some
manner, or is this a bug that keeps them around despite the fact that the
database row for it has been removed?

David