When we moved into our new house I saw a faint water spot on the ceiling and made a note to keep an eye on it. Couple of days ago the water spot appeared more noticeable and wet to the touch. Today I opened up the ceiling, but everything is completely dry. In the attached photos the water spot is directly below the elbow connecting to the sewer line, the u-trap is connected to the toilet. I flushed the toilet above couple of times, even let the water run through without collecting in the tank - no apparent leaks. Any ideas what might cause this water and spot and what else should I try? I should note that there is a 2x4 to the left of the pipe that touches the ceiling and water spot continues there as well, may be I should open up ceiling there as well? Thanks for your help.
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.
Normally, I would expect to see indications of water dripping from the bottom of the pipe for me to suspect a leak from that pipe. I see no such indications. What I do see is indications of water traveling along side of and puddling next to the afore mentioned 2x4. From above follow the water staining.