Can't figure out where leak is coming from
Leaks are almost never directly above the wet spot. They are somewhere above the wet spot (water does not run uphill), but can be a long horizontal distance if there's a way for the water to run along.

If you suspect the toilet, dye is one way to check, but presumably the toilet has been there all along, and if, say the recent reappearance of the wet spot coincided with wet November weather, I'd be looking at the exterior surfaces of the house and water intrusion there more than the toilet, unless it got clogged and overflowed recently, or the bathroom was enthusiastically wet-mopped, or tub-splashed, or something like that. 

Could, for instance, be the flashing on the vent pipe letting rain down the outside of the sewer pipe. Could be a roof leak or window flashing leak running along and dripping off here. Well-nigh impossible to diagnose with as little as you've looked at so far, but where-ever it is, is up. Just might be 20 feet sideways, too.