Toilets do not flush unless you push down the handle on the outside of the tank. When you hear what sounds like your toilet flushing, even though nobody is in the bathroom, your toilet is not part of a paranormal phenomenon. Correcting the problem requires you to find the leak in the toilet's tank, which causes the flushing noise.
The phenomenon of a toilet sounding like it flushes by itself is called phantom flushing. In reality, what you hear is the toilet replenishing some of the water in the tank, which slowly drains out over time because of a leak. The flushing sound is the water coming through the tank's fill valve, which always runs after you flush the toilet, refilling the tank with water.
The frequency a toilet phantom flushes depends on the severity of the leak in the tank. As the leak increases, you hear the phantom flushing more often. Each time you hear the tank partially refilling, you are hearing your toilet wasting water. Phantom flushing leads to a higher water bill each month as your usage increases even though your water-use behaviours have not.
You must find the source of the toilet tank's leak before you can do anything about phantom flushing. Remove the cover from the toilet's tank and carefully set it carefully. Wait until the toilet is completely silent and the water is still, then drop three or four drops of dark food colouring in the tank's water. Closely watch the food colouring as it gravitates to the leak. For further confirmation, the coloured water is visible either outside the tank or in the toilet bowl.
If the food colouring leaks past the flapper in the bottom of the tank and ends up in the bowl, check the chain to ensure it is not bound up. Flappers wear out over time, but removing the old one and installing a new flapper only takes a few minutes. If the food colouring flows past the bolts or the fill valve in the bottom of the tank, tighten the leaking parts before replacing them. If you find the food colouring exits the tank through a crack, replace the tank with a new one.