Haven't read the whole manual...yet...but, in the meantime, try this: with the whole schmear running (live band, canned track, whatever), put your headphones on, solo the ghost channel, and listen to and identify whatever the heck is coming through the 'phones. Feel free to boost the trim to saturate the channel if necessary.
Stupid question: does the Mackie allow you to re-assign certain functions (aux's, etc.) to an empty mic channel internally or does everything require cabling?
BTW, if you're wearing the 'phones and hear "...get out!...", heed the warning and bolt. You are not obligated to share this with your bandmates.
"...210 lbs. of twisted steel and sex appeal..."