1. A device comprising: an RFID reader that reads a plurality of RFID tags in an area comprising a plurality of locations and calculates a number of the RFID tags that are in each of the locations; and a display that: displays an image of the area, and overlays a numerical value representing the number of the RFID tags within each of the locations over the respective locations displayed in the image, wherein the display displays a scan trace overlaid over said image; wherein the scan trace is provided by a line comprising a plurality of time varying points, each point being defined by a projection onto a chosen plane at a given moment in time of a coverage shape of an RF signal transmitted by the RFID reader.