It's hard to find what you're looking for in the dark. Instructables users Jayefuu and Kiteman developed a way to light up a small area whenever you call a cellphone.
The setup uses a battery-powered strand of lights, a basic circuit board with a phototransistor, and a cheap cellphone with the capability to light up its screen when called. When you call the cellphone, the backlight comes on and activates the phototransistor, and that turns the lights on. They developed this for use at music festivals so it's easier to find your sanctuary in a sea of tents, but you can use it for anything. Find your car in a giant parking lot at night, light up your porch when you get home, or anything else you can come up with. For a complete guide to making the setup, check out the link below.
Festival Tent Finder | Instructables via Hack a Day