From "Wireless Keystroke Logger Disguised as USB Device Charger Targets Wireless Keyboards":
http://www.americanbar.org/content/dam/aba/administrative/cyberalert/keysweeper.authcheckdam.pdf
Technical Details
KeySweeper is an Arduino-baseda device which is contained within the
shell of a USB phone charger. It is capable of detecting and decrypting
radio frequency (RF) signals from certain Microsoft wireless keyboards
manufactured before 2011 (many of which are still available for
purchase).
The information in this notification was obtained through an FBI investigation and is provided in conjunction with
the FBI’s statutory requirement to conduct victim notification as outlined in 42 USC § 10607
Hermann.