1. A method of monitoring a scene, the method comprising: receiving image data representing a thermal image captured by a thermal image sensor and a visual image captured by a visual image sensor, the thermal image and the visual image depicting a same view of the scene; detecting objects which have a different temperature than their environment, such as human beings, other living creatures or motor vehicles, in the visual image and in the thermal image; defining a mask covering a window in the view of the scene; matching the detected objects between the visual image and the thermal image in order to determine if an object or an object part, inside the mask, is present in both the thermal image and the visual image, and, if so, providing a modified mask by excluding an area occupied by the object or the object part from the mask.