If you use an older wireless keyboard (does it have a 'Connect' button?), it's pretty easy to intercept the keystrokes without accessing the computer. In this case, the developer make it look like a USB charger.

If you use wireless keyboards – you should ensure they are encrypted with AES-128 (Logitech's keyboards do this. There is no connect button. They are paired to the receiver at the factory with encryption keys. Newer Microsoft keyboards do as well. Check online for your model).

See http://www.logitech.com/images/pdf/roem/Logitech_Adv_24_Ghz_Whitepaper_BPG2009.pdf for more information on how keyboard encryption works.

How a $10 USB Charger Can Record Your Keystrokes Over the Air
Hardware hacker and security researcher Samy Kamkar has released a slick new device that masquerades as a typical USB wall charger but in fact houses a keylogger capable of recording keystrokes fro…