I found a random on the internet using a preprint study of Barcelona sewage samples that shows that SARS-CoV-2 was in people last March (at least in Barcelona). I was immediately skeptical of this claim, because I think we would have noticed by other means if that was the case, and the theory in the paper is that it was masked by seasonal flu, which seems implausible to me. The random was using the paper to claim that the virus is nothing to worry about, "stands to reason, if it's been around that long, innit" (not actual quote, but should give the impression of what was said).
The claim seems to rest on a small amount of Covid RNA being detected in a frozen sample of sewage. There doesn't seem to be any statistical analysis done on the key result for the conclusion that it was around last march. However, I don't know enough about the methods used to know what might have happened, apart from a possibility of accidental contamination, or if you're doing around 50 tests, a couple might give a positive by accident.
I'm hoping someone here who knows stuff can explain things to me so's I can tell someone they're wrong. I am also happy to be told they are right, but not as happy, because then I'm wrong but at least I learnt something.
The preprint is here: clicky.
SARS-CoV-2 was detected in Barcelona sewage long before the declaration of the first COVID-19 case, indicating that the infection was present in the population before the first imported case was reported. Sentinel surveillance of SARS-CoV-2 in wastewater would enable adoption of immediate measures in the event of future COVID-19 waves.