A method of detecting a metal object in a detection zone between a transmitting antenna (101) and a receiving antenna (102) installed in proximity of an automatic door (105; 106), the method comprising: demodulating an amplitude modulated signal received via the receiving antenna to provide a first signal; and on a recurring basis: detecting a first base level of the first signal at a point in time when the first signal is settled at a magnitude at or about the first base level, and following a first signal transition of the first signal, detecting a second base level of the first signal at a point in time when the first signal is settled at a magnitude at or about the second base level; predicting a lower base level to be the lowest of the first base level and the second base level and setting a first threshold value, which is set relative to and lower than the lower base level predicted; and evaluating whether the first signal falls below the first threshold value and setting a first alarm detector at a point in time, while the first signal is below the first threshold value. Thereby it is possible to detect a metal object, such as a ‘booster bag’, in a detection zone in proximity of automatic doors even at the very moment the automatic door opens.