About two months ago a section of my kitchen cabinets started to receive a wet spot at the top or ceiling of the cabinet. I cut a hole in the drywall above the cabinet to see where the water was coming from. At first glance I thought it was condensation from Freon lines that I could see through the drywall hole, but the line was dry during the day. The leak upstairs only seemed to get going in the evening when taking showers or using the toilets upstairs. It was a very small leak, but seemed to travel down the Freon line. I tried caulking our two tubs and installed new tub drains, but the issue continued. It was only when I discovered our downstairs toilet was running a few times a day and turning off the water to that toilet completely did the leak upstairs stop. Yesterday I installed a new flapper and the toilet is not running at all. Why would a running downstairs toilet cause a leak upstairs? Should I add new wax rings to all of our toilets? Biggest question: Is it possible my main house drain line is partially blocked causing a small leak in the second floor? If it is the main drain line, should I remove some drywall to attempt to locate the leak?