Is my postgres index being used?
The documentation states the following:


  Indexes can be used via either simple index scans or "bitmap" index
  scans. In a bitmap scan the output of several indexes can be combined
  via AND or OR rules, so it is difficult to associate individual heap
  row fetches with specific indexes when a bitmap scan is used.
  Therefore, a bitmap scan increments the
  pg_stat_all_indexes.idx_tup_read count(s) for the index(es) it uses,
  and it increments the pg_stat_all_tables.idx_tup_fetch count for the
  table, but it does not affect pg_stat_all_indexes.idx_tup_fetch.
  
  Note: The idx_tup_read and idx_tup_fetch counts can be different even
  without any use of bitmap scans, because idx_tup_read counts index
  entries retrieved from the index while idx_tup_fetch counts live rows
  fetched from the table. The latter will be less if any dead or
  not-yet-committed rows are fetched using the index, or if any heap
  fetches are avoided by means of an index-only scan.


This gives you two possibilities.  The first one you've excluded by rebuilding the index, so one could guess this index is used exclusively via index-only scans and/or bitmap index scans.  You can confirm this buy checking the EXPLAIN output of your queries that may use this index.

You can read a bit more about bitmap scans on depesz's page.