What you want is an RFID keyfob/detector. There used to be a writeup on a homebrew version of this at http://hackaday.com/2013/09/07/hidden-rfid-reader-locks-work... -- but the link that article references is rotten now. A sufficiently motivated person should be able to dig up instructions somewhere.
A few years ago there were companies that did this for most operating systems, and fairly inexpensively too, but I'm having trouble finding them now.
A few years ago there were companies that did this for most operating systems, and fairly inexpensively too, but I'm having trouble finding them now.