Proximity sensor to keep session active

I am looking at a PCI compensating control to keep a workstation session active if staff are located in very close proximity to the workstation. I was told that there may be a proximity sensor device that plugs into the USB port on the computer and the user would then carry a device that communicated with the with the sensor to keep the session from a network enforced screensaver. Any advise on this matter would be much appreciated. I've tried a Google search and found some devices that would lock out a terminal, but not keep one to keep the session active.