By default, the map file is called pg_ident.conf (because it was first used for the ident authentication method).
Nowadays, it is possible to change the name of this file via the ident_file configuration parameter in postgresql.conf. It can also be located outside the PGDATA directory by setting ident_file to a full path.
A relative path can also be used, but since it is relative to where the postgres process is started, this is usually not a good idea.