Survivorman & Survivorman/Bigfoot star Les Stroud shared some “unknown” audio he recorded and sent to OP audio specialist David Ellis for analysis.
According to David:
I use “suspicious” when referencing a vocalization. I go through the process of disproving known animal vocalizations using the Macaulay Library of sound. I look at the spectrograms trying to find a visual match. When I can’t find a match the recorded vocal becomes suspicious until I can prove otherwise
In science we are following the terms of the null hypothesis. We assume every sound has a known origin. When you eliminate each known your case for unknown gets stronger.