https://academic.oup.com/cid/advance-article/doi/10.1093/cid...
Edit: removed incorrect conclusion
https://www.reuters.com/article/us-health-coronavirus-spain-...
Kept searching for followups, didn’t see anything till this:
https://www.sciencedirect.com/science/article/pii/S004896972...
That’s from 21 January 2021 and says:
“So far unpublished data, analysing retained wastewater samples from Barcelona, Spain, taken in March 2019 showed positive RT-qPCR results for SARS-CoV-2 (Chavarria-Miró et al., 2020). The authors conclude that the virus was introduced to the Barcelona population due to global travel and remained undetected.”
So as of then, still unpublished.