If no clients are connected and transmitting to an AP, is it still possible (or are there any scenarios) to have co-channel interference if the AP is operating at 2.4GHz?
Sure. Other AP's on the same channel - if nothing else, they are emitting Beacons . Also client devices attached to these other AP's.
If you want to get into interference, like Microwave Ovens, etc. It can be even worse.
Same for 5 GHz, but less likely as, in general, it's less crowded..