At any given moment here on Earth, there is always someone, somewhere scanning the skies for a hint of alien life. We have telescopes all over the world, silently and patiently waiting for the merest blip from an extraterrestrial. We have also sent literally hundreds of little robots off out into the solar system, searching high and low for the merest hint that we aren't all alone in the universe.
So far, in all our years of listening, we've come up with precisely nothing.
Even so, although we have so far failed to pick up a friendly hello from a passing alien life form, there have definitely been times when we thought we had. These days, news of a signal from the stars that is "probably almost definitely aliens this time" has become a surprisingly common occurrence. Generally speaking, after the media hype settles down, we tend to find that the signal was actually originating from a black hole, pulsar, bit of space junk, or even the staff break room at the observatory (no, really).
There have been a couple of times, however, when these signals from space have gone unsolved. This means that, so long as we haven't pinned down a definite source, they still could be the hailing beacon from the hyper-intelligent little green men in the next galaxy.