OP means that while developing the radio and antenna part of the fob the engineers have adjusted the radio and antenna to work the best when in close proximity to a body. The human body has a certain effect on the radio signals, so adjusting the radio and antenna to work best while close to the human body will mean that it won't work as optimally when further away from a human body.

It doesn't detect the human body, it is just designed to work the best when it is physically close to the human body.