  We report the detection of cold dust in an apparently quiescent massive
galaxy ($\log({M_{\star}/M_{\odot}})\approx11$) at $z\sim2$ (G4). The source is
identified as a serendipitous 2 mm continuum source in a deep ALMA observation
within the field of Q2343-BX610, a $z=2.21$ massive star-forming disk galaxy.
Available multi-band photometry of G4 suggests redshift of $z\sim2$ and a low
specific star-formation rate (sSFR), $\log(SFR/M_{\star}) [yr^{-1}] \approx
-10.2$, corresponding to $\approx1.2$ dex below the $z=2$ main sequence (MS).
G4 appears to be a peculiar dust-rich quiescent galaxy for its stellar mass
($\log({M_{\rm dust}/M_{\star}}) = -2.71 \pm 0.26$), with its estimated
mass-weighted age ($\sim$ 1-2 Gyr). We compile $z\gtrsim1$ quiescent galaxies
in the literature and discuss their age-$\Delta$MS and $\log({M_{\rm
dust}/M_{\star}})$-age relations to investigate passive evolution and dust
depletion scale. A long dust depletion time and its morphology suggest
morphological quenching along with less efficient feedback that could have
acted on G4. The estimated dust yield for G4 further supports this idea,
requiring efficient survival of dust and/or grain growth, and rejuvenation (or
additional accretion). Follow-up observations probing the stellar light and
cold dust peak are necessary to understand the implication of these findings in
the broader context of galaxy evolutionary studies and quenching in the early
universe.
