What to do about a “phantom flush”:
On occasion, you may hear your toilet flush itself. Though this can be disconcerting to hear in the middle of the night, it’s no ghost and no home intruder. Instead, the phantom flush indicates that water is leaking from your tank into the toilet bowl. Thanks to the physical laws that make toilets work, when enough water fills the bowl, it flushes automatically.
In most cases, a wornout flapper in the tank causes this leak. Test for a leak by putting a few drops of food coloring in the tank and waiting to see if it becomes visible in the bowl without flushing. You can hire a plumber to replace the flapper, or if you feel handy enough, you can purchase the parts at a hardware store and try it yourself.