HELP ! I live in a 2 story house. I noticed a brown spot on main floor dining room ceiling (around a light fixture) It feels damp and spongy. I have not had any flooding in the upstairs bathroom and no noticeable leaks. There are no windows in the upstairs bathroom or on that side of the house at all.

Is the upstairs bathroom located above the dining room? Just trying to cover the bases here.

First thing to check is as CLOUGH said... is the bathroom directly above the dining room? If so, I would start by checking the obvious stuff. Does the toilet move? I mean, when you put your hands on toilet can you shift the toilet... does it move or rock at all.. If so, probably need to lift toilet and replace the wax gasket. Also, check the shutoff and where the toilet meets the tank. Next place to check is under the sink.. any moisture there... look closely at the shutoffs... especially near the shutoffs... USE TOILET PAPER to actually wipe the shutoffs and confirm no moisture present. The next place to check is at the tub/shower valve... can check behind the cover plate by removing screws from faceplate and looking behind wall (use flashlight here) while tub and THEN shower are running... sometimes diverters can leak behind wall... sometimes shower arms corrode, etc.

Netx thought is to check that any teenagers are actually pulling shower curtain over during shower.. I have seen this more times than my customers would like to admit!!

Also, check the radiator or baseboard radiation (if you have them). Here, a packing nut on top of valve (radiator) or even a coin or key vent could be leaking under the baseboard cover.

After that.. well... check back with us and let us know what you found. Next steps are to measure and explore other options.