Sounds all to familar, especially with people not locking their workstations. At a previous employer we had a security audit for anyone working on sensitive high level information and if you worked on the Ministry of Defence contracts, to which we where given a sonar to put on top of our monitors, so that if you moved so many inches away from your workstation it automatically locked the machine for you.

http://www.rfideas.com/products/presence_detector/pcprox_sonar/