I have a toilet that will start running as if the flapper is bad. (I’m confident it is not. I’ve replaced it twice.) It doesn’t drain much, just enough to trigger a fill every 20 - 30 minutes.
The strange thing is the tank only drains slightly and then stops. If I turn the water off it will only drain approximately 1/4 to 3/8’s of an inch and then stop draining completely. I can leave the water turned off for two weeks and it will never go below that level. I’m thinking if it were a bad seal it would drain completely after that time.
Is it a crack in the tank? I’ve just about had it and ready to replace the toilet. It’s approximately 20 years old.