Think about the protracted debate concerning “Tabby’s star,” which is showing strange brightening and dimming. Some say it’s due to a possible “alien megastructure,” while others are pointing to various natural processes, such as exocomets.
“More structure is obviously necessary, as far as signals are concerned,” Cirkovic said. “The example often quoted (first due to Nikola Tesla, who was somewhat forgotten as a SETI pioneer) is the presence of prime numbers 2, 3, 5, 7, 11, … etc. in the signal.”
Prime numbers are a non-natural process that are considered by SETI scholars an indication of possible life.