Here on Earth, we make it very obvious that a (mostly) intelligent species lives here. We broadcast signals on nearly every wavelength imaginable, and one of those is visible light: every night, we inform the universe of our presence through our rampant light pollution, and extraterrestrials may be doing the same thing.
Astronomers from Harvard and Princeton have cleverly pointed out something that should be obvious: unless extraterrestrials live in some kind of exotic binary star system where it's never dark out, they've probably have some kind of artificial illumination that they turn on at night to keep their flying saucers from getting into accidents.
The thing about artificial illumination is that it's artificial, and anyone with a telescope and a moderate amount of cleverness can tell the difference between light from a star and light from thermal sources (like incandescent bulbs) or quantum sources (like fluorescent lights, lasers and LEDs). Also, we can just compare the amount of light that the planet reflects depending on where in its orbit it is, and if there's light coming from the planet that's not coming from its star, we'd notice.
Of course, we've already got other ways of looking for extraterrestrial intelligence — radio waves, for one — but here on Earth, as our technology advances, the amount of radio emissions we produce has been declining drastically, since things like fiber optics work way better. It's entirely possible that extraterrestrials, who are likely to be significantly more advanced than we are, aren't in the habit of blasting out radio frequency tunes anymore, which would make them harder to spot. But unless they can see in the dark, they'll still need lights.
In order for us to detect extraterrestrial streetlights at our present level of telescope technology, the brightness of the lights would have to be comparable to the brightness of the planet when lit by its star, which is about 100,000 times more light than we put out at night on Earth. So either we need some more sensitive telescopes, or ETs need to be hanging out in the Kuiper belt out past Pluto, which seems unlikely. But we're getting better at seeing farther out in to space in more detail than ever before, and now we've got something else to look for.