WAL sender process starts when streaming client is active. The connection is initiated by the client. That's why you cannot see it. It also uses the TCP-port that is used by any other pg client. There is a special entry in the pg_hba.conf for the replication clients. Refer to this please: http://www.postgresql.org/docs/9.4/static/auth-pg-hba-conf.html.
cheers
Matthias
