Ok. Found the place. So, there is nothing there. PG_DATA/base/pgsql_tmp is a empty directory. I've run the query over pg_stat_database view and there is nothing wrong with pgAdmin III - the information is all there. I've also run a vacuum freeze analyze, but made no difference.
I believe that statistics are outdated, since there is no temp file at all.
The statistics are not current, they are cumulative since the database was created, or pg_stat_reset() was last called.