The method uses at least one transmitter (3) and at least one receiver (4) coupled to an evaluation stage (20). The transmitter beam (5) is used to scan a reference object within the surveillance zone a number of times, for providing the contours of the reference object, the corresponding position values used to provide reference position values. The object is detected by providing difference values between the actual position values and the reference position values, which can be evaluated via a number of tolerance bands.
