OK, here's what I got from the debug window.
-> efnet.xs4all.nl QUIT :tester
<- ERROR :Closing Link: adsl-209-78-18-72.dsl.renocs.pacbell.net (Client Quit)
This same server, only minutes earlier, displayed a quit message for one of my friends running a different client. My quit message (as seen by several other friends) was "Client Quit". I also noticed that when you disconnect by closing mIRC itself, it quits with your default quit message (again, as seen by friends online during testing). Perhaps the old "soft disconnect" feature, even though now removed, is causing this (lines of code left in the source?).