I think the most useful for you would be Proximity; it can run any AppleScript when you're within a certain distance of your computer.
-OR-
You can take a look at BlueProximity (should be compile-able for OS X if there are no binaries available). Instead of getting it to lock your screen/log you off, you could set it to, for example, show a Growl alert when you are within a certain distance of your computer.