This is a pen. It costs $20 and can detect wireless signals using a series of between one and four glowing lights. The more lights you see, the stronger the signal!!! It runs off of two AG13 batteries, which look like tiny little buttons. I’d hope that you can somehow replace the ink once it runs out because, man, that would be a bad day.

WiFi Signal Finder Pen [Gadget4all.com] via Pocket-lint