A system and method for short-range ultrasonic detecting of a target is proposed herein to be used for example in a liquid-dispensing system such as those provided in home refrigerators. The system comprises a sensor assembly including a transmitter and a receiver positioned side by side so as to yield respective transmitting and receiving beams which overlap to define an asymmetrical shaped detection area, a controller coupled to both the transmitter and receiver to activate the transmitter and for receiving echoes indicative of the target from the receiver and to the dispensing system for opening a valve of the dispensing system when a target is detected. Noises and false alarm rate minimization methods are implemented, including variable relevance zones and requiring a void of signal before excitation.