missing listener.ora file

 I've inherited a few databases that I need to manage.  A few of them don't have a listener.ora file.  lsnrctl status does not show a listener.ora file and there is no listner.ora file in the $ORACLE_HOME/network/admin directory.  I need to turn on  TRACE_LEVEL_LISTENER=user

But without a listener.ora file I don't think this is possible.  So my question is why would someone configure Oracle NOT to use a listener.ora file?  It seem an unusuall setting; one that wouldn't have been done by accident.  So I'm wondering if I create a listener.ora file will I be breaking something?  And by the way, how did they set the database to not use a listener.ora file -- is there some parameter setting that needs to be set somewhere or does it just dynamically register in the absence of the listener.ora file.   --Thanks.