Try using
Code:
on *:NOTICE:*This nickname is owned*:*:{

On 3 of the 4 networks that I'm on, that notice actually reads "This nickname is owned and protected", only on the 4th does it actually read as yours indicates. Also, nickserv notices appear in my status window, so they aren't recognized as being via pm, which is what the ? at the end indicates as the location for the message, so I use the * that way it gets caught no matter where the message actually shows.