  The aim of the search for extraterrestrial intelligence (SETI) is to find
technologically-capable life beyond Earth through their technosignatures. On
2019 April 29, the Breakthrough Listen SETI project observed Proxima Centauri
with the Parkes 'Murriyang' radio telescope. These data contained a narrowband
signal with characteristics broadly consistent with a technosignature near 982
MHz ('blc1'). Here we present a procedure for the analysis of potential
technosignatures, in the context of the ubiquity of human-generated radio
interference, which we apply to blc1. Using this procedure, we find that blc1
is not an extraterrestrial technosignature, but rather an
electronically-drifting intermodulation product of local, time-varying
interferers aligned with the observing cadence. We find dozens of instances of
radio interference with similar morphologies to blc1 at frequencies
harmonically related to common clock oscillators. These complex intermodulation
products highlight the necessity for detailed follow-up of any
signal-of-interest using a procedure such as the one outlined in this work.
