Theoretically, this would also trigger if the channel-name itself contains a ?, although I do not remember if the rfc1459/irc protocols do support this. Of course, your eggdrop would have to join such a channel in the first place to receive messages from such channels..
Strictly however, you should use something like below to have the behaviour you described:
Hmm, that's what I thought. I found the form you mention as well, and neither seem to be working for me. Nothing's coming up in any logs or in the console itself. It only seems to fire when the ? is the *only* thing on the line.