A Tentative Emission Line at z = 5.8 from a 3 mm Selected Galaxy

I report a tentative ($\sim4\sigma$) emission line at $\nu=100.84\,$GHz from "COS-3mm-1'", 3mm-selected galaxy reported by Williams et al. 2019 that is undetected optical and near infrared wavelengths. The was found in the ALMA Science Archive after re-processing band 3 observations targeting different source. Assuming corresponds to $\rm CO(6\to5)$ transition, this detection implies spectroscopic redshift of $z=5.857$, agreement with galaxy's constraints multi-wavelength photometry. This would make object highest one dusty star-forming galaxies known to-date. Here, characteristics physical properties can be inferred assuming real. Finally, advocate for follow-up corroborate identification confirm high-redshift nature optically-dark galaxy.