Disc space usage

One of our build servers recently ran out of disc space while trying to
copy an entire database. This led me to investigate the database cluster,
which is stored on a RAID array with a total size of 1TB. Running a query
to list all databases and their sizes did not add up to the amount of
space being used by Postgres, so I had a look at the pgsql/base directory.
It appears that there are a few large directories that do not correspond
to any database. I wonder if these have been left behind accidentally by
Postgres.

Here are the database directories:

  Size (kB)    Directory    Database

  32        pgsql_tmp
  4352        11510        template0
  4368        1        template1
  4464        11511        postgres
  5368        30103627    xav-userprofile-test
  6096        8088167        outerjoins-userprofile-12.0-copy
  8676        30103406    xav-test
  10052        31313164    common-tgt-items-kmr-modmine
  19956        1108178        modmine-3-preview-18-feb-2008
  89452        14578911    common-tgt-items-kmr
  118940        9952565        production-xav-13
  201192        1257481        common-tgt-items-gtocmine-rns
  296552        7040137        common-tgt-items-flyminebuild
  1557160    9843085
  1699624    18456655    common-src-items-flyminebuild
  3376096    278561
  3995276    9064702        production-unimine-pride-beta5
  8528136    1257482        gtocmine-rns
  40815456    29233051
  42278196    27473906
  47112412    28110832
  47913532    32728815    production-flyminebuild:ensembl-anopheles
  60519524    32841289    production-flyminebuild:go
  67626328    27377902
  69513844    32856736    production-flyminebuild:flybase-dmel-gene-fasta
  74289908    32938724    production-flyminebuild:pubmed-gene
  75786720    32941684    production-flyminebuild:biogrid
  77361800    32944072    production-flyminebuild:update-publications
  80160256    32947141    production-flyminebuild:create-references
  81333908    32574190    flybasemine-production
  86356140    12110825
  87544200    33049747    production-flyminebuild

So on this server, the wasted space takes up 276GB, which is not
acceptable. I believe that if we re-initialise the cluster and re-create
the databases, these directories would disappear. Taking a look at the
directory 12110825, all the files inside were last accessed several
months ago. So, I have a few questions:

1. Is this space used for anything, or is it just abandoned? Is this a
    bug?
2. How do I reclaim this wasted space in a safe manner?
3. How do I prevent this happening again?

Matthew

--
An ant doesn't have a lot of processing power available to it. I'm not trying
to be speciesist - I wouldn't want to detract you from such a wonderful
creature, but, well, there isn't a lot there, is there?
                                        -- Computer Science Lecturer