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 2×4 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.