It sounds a little ghetto, but hear me out:

Take wire screen about 1 ft x 1 ft, and glue crumpled aluminum foil to
it. Then unplug the router.

Take the screen and place it in between the antenna and the area you
think the signal is coming from.

Boot the router back up, and see if the user comes back online.

Tweak this until the user doesnt come back.

Essentially you're building one side of a Farraday cage. It shouldn't
allow RF through, and should help you pinpoint the signal.

Chris