When a Bluetooth device comes in range of your Mac, the computer can unlock without user interaction by the Lifehacker guide. Also added tag is that it locks the system when the Bluetooth range is out. We do not require any app or special Bluetooth device as such to operate that.
Proximity is a utility for Mac required by the guide with a few simple AppleScripts at MacWorld Hints. Setting it up and the process can be downloaded along with a user help file is anything goes wrong. The entire process is very fast and can be saved as a screen saver to check if the Mac is locked or unlocked. This saves time of logging in and logging out each time u need to leave the place. But it may be a hindrance for security issues. Otherwise it’s a very useful idea and should give it a try!
[Image courtesy Lifehacker]