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I believe that statistics are outdated, since there is no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.