A phantom toilet flushing (when the toilet randomly runs for a few seconds on its own without anyone pressing the handle) is almost exclusively caused by a slow internal water leak from the tank into the bowl. The most common culprits include a warped, pitted, or rigid rubber flapper valve that fails to seal the flush valve seat tightly, a corroded brass flush seat covered in mineral scale, or a fill valve float set too high that allows water to constantly trickle down the overflow tube. Verification: Add dark food coloring to the tank; if colored water seeps into the bowl within 15 minutes without flushing, an internal leak is present.
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