Mankind constantly analyzes radio waves from outer space in the search for extraterrestrial intelligence. Since this analysis started, almost all of the signal sources have been identified. 37 signals, however, remain unexplained.
The 37 number comes from a single 1993 paper on a search programme for extraterrestrial hydrogen line beacons. Unless you believe that nothing has happened in radioastronomy for the past 31 years, there is no reason that this number remains 37 today. This page is either clueless or firmly tongue-in-cheek.
https://37signals.com/37