I too have noticed these PONG events occuring since my upgrade to mIRC 6.17. Here are the finds of my investigation:
Normally, the server itself sends PING commands in certain intervals to each of its clients to determine their presence. mIRC properly answers these with the PONG response.
<- PING :irc.inet.tele.dk
-> irc.inet.tele.dk PONG :irc.inet.tele.dk
The problem I experienced is caused by the following PING command, which mIRC occasionally sends to the server:
-> irc.inet.tele.dk PING :TIMEOUTCHECK
<- :irc.inet.tele.dk PONG irc.inet.tele.dk :heidel
Why mIRC sends these I have no idea, I can only speculate. Here's my theory:
I use an IRC bouncer named psyBNC and this bouncer does not forward the server's PING messages to the client but answers them itself.
But since there is constant traffic on my IRC connection from the bouncer, one would assume, mIRC considers the connection "alive". However, every few hours mIRC sends the aforementioned PING command to the server. And the following PONG response is obviously not being affected by the "Hide Ping? Pong! event" option.
Also, I have no idea as to why this has never happened to me with mIRC 6.16.
Here's a scripted workaround to keep mIRC from outputting the default PONG event text: