Wi-Fi device detection

Walk around the area you suspect the device to be and take a map. Mark the signal strength at various points as you walk around. You you should be able to get very close to locating the spot.

In addition, I used to use a software package (on linux systems) called kismet. Not all wifi networks advertise there SSID, so you're lucky you can see what you can. Kismet would grab ALL wifi packets and find the SSID. It was a great tool.

Actaully, it appears kismet is still around

https://www.kismetwireless.net/
I had it on an iPac, running debian linux with an orinoco pcmcia wireless card. I used it to map every wireless network transmitting packets across the university I worked at. Very useful tool.