I could think of some signals sent through FX A or FX B to Mains, but as they are post fade this is probably not the reason. So your problem looks like a typical fader problem (how old is your board, how often has it been used, has there been some dust/moist/ice...?), which means that the fader banks need to be replaced. You may want to check, which channels "bypass" the faders. If I remember it right one fader bank is 8 channels (1 to 8, 9 to 16, 17 to 24, Groups & Main). But I would recommend to replace ALL faders, because they were a weak part in the classic mixers. The faders could easily get dirty inside, which causes misfunctions (like letting signals go through). Replacing all faders is affordable and gives you some more good years with your board.