Loosing water

The plastic overflow tank needs to be topped up with about 1.5 pints of water after approx.20mls. I cannot establish where the water is going.No indication of droplets on oil dipstick and no water blowing from the exhaust. There is no indication of overheating, temp.gauge remaing at midpopint. After the car has had a 'run'and with engine still running, hot water is observed being pumped into the plastic'overflow' tank. But following this when parked up overnight, water will drain out of the plastic tank and needs to again be filled with approx.1.5 pints of water. Although parked over a perfecttly dry surface there is no indication of a water leek on the floor. Totally confused!!!