The Smart ID WFS-1 Wi-Fi Detector rocks: I finally received my order of this slightly hard-to-get Wi-Fi sensor. It's substantially better in several angles than the Kensington WiFi Finder. First, it's instantaneous: hold down the button, and signals flash. Second, it has more common, easier to change batteries. Third, it's highly directional. Couple the directionality with the instantaneous metering and you can hunt down signals.
Now on the flip side, the documentation says it detects all kinds of 2.4 GHz radiation, so areas with no hot spots but other kinds of activity (microwaves, Bluetooth, cordless phone, and non-Wi-Fi wireless) will trigger its lights, too.
You can buy these units from Directron, a very communicative and responsive online merchant. When I initially ordered a few weeks ago, they were backordered, and they told me about the wait and confirmed my desire to wait. They also emailed me when the units became available and shipped them quickly after that.