Phantom toilet flushing is caused by a slow internal water leak from the tank into the bowl, almost always resulting from a degraded, warped, or calcified rubber flapper valve. As water steadily drips past the imperfect flapper seal, the water level inside the tank gradually recedes. Once the water level drops below the critical float trigger threshold, the fill valve automatically opens for a few seconds to refill the tank back to its proper operating height, creating the illusion that the toilet is flushing itself. Verification: Add dark food coloring to the tank; if colored water appears in the bowl within 15 minutes, the flapper is leaking.
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