Contents of data/base/<oid> and no corresponding entry in pg_database

Hi,

(We're running postgres 9.2.X)

from reading
http://www.postgresql.org/docs/9.2/static/storage-file-layout.html the
directories under $PG_DATADIR/data/base should correspond to an actual
database. I've however found a few directories in $PG_DATADIR/data/base
where select datname from pg_database where oid = <oid> returns 0 rows.

The machine does have "high churn" on databases and have suffered a few
crashes, is it possible for things to get left behind in
$PG_DATADIR/data/base after recovery ? or am I possibly missing things
still in use, that's reflected elsewhere ?

Additionally, the directory and it's contents haven't been touched in quite
a while as well, and some of them do correspond to being touched at the
machine having had a hard crash.

The reason I'm asking is that space used as reported by pg_database_size is
off by a LOT compared to df on the filesystem (4-5TB as oposed to df/du
agreeing on 9-ish TB).

Cheers,
--
Kjetil Jørgensen <kjetijor(at)gmail(dot)com>