Well, the question was about discovering listener itself and not the databases it serves.
I don't know if 10g is different then 9i, but in 9i the instances need to be explicitly defined in the listener.ora file. The agent parses the listener.ora file using the information gained from the oratab file. If the database is not defined in the listener.ora file, it will not find it.
-- Best regards, Alex Gorbachev
http://blog.oracloid.com -- //www.freelists.org/webpage/oracle-l