Have you notice the mIRC quit message in the triliant window ?
Also there can be a bug in some script(i hear some kind of bug in PnP script, but don't know sure what exploitation allow to do).
So, why someone need to close your client if he have(if that is some exploit etc, bug in script,etc) full control and can just op himself ?
Ehehe.. Maybe smth else..
If you'll leave again your mIRC for a night, you might type /debug smth.random.txt in mIRC, so if
that happens again, you will know sure what was the last irc-activity!