Strange Signal
Astronomer here! In my field, the Wow! signal is probably the most famous unexplained mystery. It was basically detected in 1977 by a telescope that was just pointed overhead, and looked very similar to what a SETI signal is expected to look like.
It was first noted the next morning by an astronomer (who wrote “Wow!” in the log margins) and was never seen again despite extensive follow up over the years. There have been various suggestions on what the Wow! signal was (but no, it probably wasn’t comets).
Personally, based on my experience with man made interference (RFI) in radio astronomy (I wrote a paper on distinguishing a real transient signal from that stuff), it was most likely RFI from a satellite or similar.
But until we see another signal like it, we just don’t know for sure, and that’s very fun to think about!
Story Credit: Reddit/Andromeda321