There's a difference between paranoid for no reason and paranoid because someone's standing behind you right now as you're reading this. Good thing for both types this Auto Detective Pen only costs $17.71. The pen scans your surroundings for unknown wireless signals, and when found, lights up automatically. Now for some Engrish by someone other than me:
The color light flash will be stronger and stronger when the detector is nearer to the suspicious transmit, then you can touch or hold the cap of a pen to reduce the sensitivity of the detector, until find the source of the signal
It apparently also scans money to see if it's counterfeit, but it's doubtful that you'll get an accurate read from a $17 toy.