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I assume that since you are using an AirPort Extreme you have Macs at that office. The easiest way to map out the WiFi coverage is to use software that lets you draw a floorplan, then take a MacBook around the building to get multiple signal strength readings. NetSpot is pretty good for this. There are probably other good options, but NetSpot is the only one I’ve used. When you are done, you should be have a comprehensive site survey showing the WiFi performance throughout the building, where any dead zones are, and be able to identify and address problems caused by things like plumbing or building materials that are blocking the signal.