Sniffing off the air NRF24L01+ and Bluetooth LE packets for under $30
blog.cyberexplorer.me
blog.cyberexplorer.me
BUT: This has been done years ago for under $1 (the price of a nRF24L01 board) by using a nRF24L01 itself.
http://travisgoodspeed.blogspot.de/2011/02/promiscuity-is-nr...