This applescript, coupled with the OAuth shim available at https://latitudebridge.appspot.com/ allows you to populate a couple of variables indicating a device's distance to your home. Send yourself an email if your garage door opens but the user is not near by.
Modified from Instamapper script.
Just add a trigger than starts the script on Indigo start-up.
Set your home's latitude and longitude in the script.
If configured properly, you should see the variables popping up with the user's name, distance and latitude and longitude.