The following command, being issued WITHOUT specifying username and password:

./bin/fbsvcmgr service_mgr action_trace_start trc_name "p1" trc_cfg ./fbtrace_probe01.conf

-- displays only starting message ("Trace session ID nnn started") and no any other messages about activity are displayed.

Enabling logging services in trace config shows that FBSVCMGR connects in such case as 'firebird' rather than 'SYSDBA' - even when ISC_USER and ISC_PASSWORD are defined.

./bin/fbsvcmgr service_mgr action_trace_start trc_name "p1" trc_cfg ./fbtrace_probe01.conf

Trace session ID 24 started
2011-11-11T13:35:34.2800 (31731:0x7fc608043810) TRACE_INIT
        SESSION_24 p1

2011-11-11T13:35:34.2800 (31731:0x7fc608043810) START_SERVICE
        service_mgr, (Service 0x7fc60a1dccc0, firebird, internal)
        "Start Trace Session"
        -TRUSTED_SVC firebird -START -NAME p1 -CONFIG <services>
  enabled true
  log_services true
</services>
<database %[\\/](test123).fdb>
  enabled true
  log_filename /var/db/firebird/fbtrace_probe01.log
  log_connections true
  time_threshold 0
</database>

PS. 'firebird' in this case is the same name as OS user that I'm using to login in Linux