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 mobile phone.
The setup uses a battery-powered strand of lights, a basic circuit board with a phototransistor, and a cheap mobile phone with the capability to light up its screen when called. When you call the mobile phone, the backlight comes on and activates the phototransistor, and that turns the lights on.
This was developed as a tool for 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.