There was one signal that was detected in 1977 called the WOW signal, rather famous
now, because some people believe it was a message from an alien intelligence.
The WOW signal was 30 times stronger
than anything that telescopes could normally detect.
It was almost exactly in the place researchers
expected to find a signal to come from alien intelligences, at least in the
radio spectrum, and it hasn't been linked to any earth based emissions or pollution.
So, on the face of it, this looks like rather an
interesting signal, but the problem is it's never been detected ever again.
It's never been found again, and that suggests that it is an
experimental artifact, maybe an artifact of
the electronics, or the radio telescope,
or something else that we're seeing
that has no connection with extraterrestrial intelligence.