On unotify
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The on UNOTIFY event triggers when a user in your notify list leaves IRC.
on <level>:UNOTIFY:<commands>
Read access levels to get more info about the <level> field.
Example
on *:UNOTIFY: { echo -a $nick has just left IRC. }
This event would trigger when someone in your notify list leaves IRC and echo it to your acive window.
on *:UNOTIFY: { if ($nick == Dana) { msg #mychannel Dana has just left IRC! } }
This example would trigger when someone with the nick Dana leaves IRC (if the nick is in your notify list only ofcourse) and sends this information to your channel.
See Also
To react when people in your notify list join IRC, take a look at the On notify event