Here is a cool tool that alerts you when someone is coming to your home. The 1/2 Mile Driveway Alarm has a remote motion sensor that can be attached to a mailbox post or address sign at the end of your driveway to alert you of vehicle movements up to 80 feet away. It sends wireless data back to the monitor in your home.
The monitor generates a tone to let you know when someone is coming. You get an exit delay of 5 minutes to leave without generating an alert. This can also detect people coming to visit you. Not cheap but might come handy.