I was told that was ghost pressure, where you turn on water some other place and for a few seconds it releases water at the toilet. But look at it another way did you check your flappers and see if they are leaking. Take some easter dye and put in tank dont flush and see if it comes in bowl if so then replace flapper, the flapper can release water until it opens ballcock and refills.
The only way that your WCs wil “start to run” is if the tank float valves open up and will thus allow water to run into the tanks. There are several things that will cause your tank float valves to open. the most common cause is something that drains water from your tanks, and that is usually a leaky ballcock or flapper valve. This is fairly easy to identify. Put some coloring agent into the tank and see if it appears in the toilet bowl (without flushing, of course). Another problem would be a leaky or improperly set float valve shut off. This would allow the tank to fill to a point that the water rises above the tank overflow tube (which drains water directly into the toilet bowl). The color dye test will also reveal this problem. There is NO OTHER WAY, besides a leak through the ballcock/flapper valve or through the overflow tube, that water can go from the tank to the bowl without a normal “flush.”
Thank God plumbing is so logical and simple. If you think about plumbing problems logically, you can solve each and every problem, no matter how baffling it may seem. You don’t need a PhD in quantum physics to solve plumbing problems. Just common sense.