How PMON discovers service information?

Hello,
I have read that "By default, the PMON process registers service information with its local listener on the default local address of TCP/IP, port 1521."

But where PMON discovers information?
In my case, PMON launchs a correct listener DBSOURCE but an unexpected one with an unexpected status:

"Service "DBDUP" has 1 instance(s).
    Instance "DBDUP", status UNKNOWN, has 1 handler(s) for this service...

For information:  service_names = DBSOURCE and local_listener = <null>

Thanks in advance,
Armel.