We were hearing water running in the pipe to the septic tank just below the toilet in our powder room. This is what we have tried so far:
We turned off the water to the toilet at the shut off valve, flushed the toilet until the tank was empty, but water remained in the bowl.
We watched it for a day or so. The water eventually stopped running in the pipe to the septic and the water remained in the toilet bowl.
We turned the shut off valve back on and heard no water running for a couple days, but then it started again.
We put food coloring in the tank, but none leaked into the bowl.
We shut the water off, flused the toilet, marked the amount of water in the bowl and left it overnight. The bowl went down less than 3/8 inch and it did that fairly soon. The drip sound continued long after the bowl water level dropped, but the drip in the septic line stopped by morning.
We turned the water back on, flushed the toilet several times. It has been on all day and still no sound of water leaking. But from experience, we expect it will be leaking by tomorrow. It was suggested that it might be a crack in the trap, but if that were the case, wouldn't it always leak? There is no indication that the vent pipe is blocked. Any new ideas?
Shorten the flapper chain to where it just allows the flapper to close. If it is too long, it may be kinking off and on from time to time, and people usually don't notice that it won't allow the flapper to close tightly enough sometimes.
Might still be something else, but after all you've tried, check that next. Plumbing is a process of elimination (bad pun intended...lol).