  Jupiter-family comet (JFC) P/2021 HS (PANSTARRS) only exhibits a coma within
a few weeks of its perihelion passage at 0.8~au, which is atypical for a comet.
Here we present an investigation into the underlying cause using serendipitous
survey detections as well as targeted observations. We find that the detection
of the activity is caused by an extremely faint coma being enhanced by forward
scattering effect due to the comet reaching a phase angle of $\sim140^\circ$.
The coma morphology is consistent with sustained, sublimation-driven activity
produced by a small active area, $\sim700~\mathrm{m^2}$, one of the smallest
values ever measured on a comet. The phase function of the nucleus shows a
phase coefficient of $0.035\pm0.002~\mathrm{mag/deg}$, implying an absolute
magnitude of $H=18.31\pm0.04$ and a phase slope of $G=-0.13$, with color
consistent with typical JFC nuclei. Thermal observations suggest a nucleus
diameter of 0.6--1.1~km, implying an optical albedo of 0.04--0.23 which is
higher than typical cometary nuclei. An unsuccessful search for dust trail and
meteor activity confirms minimal dust deposit along the orbit, totaling
$\lesssim10^8$~kg. As P/2021 HS is dynamically unstable, similar to typical
JFCs, we speculate that it has an origin in the trans-Neptunian region, and
that its extreme depletion of volatiles is caused by a large number of previous
passages to the inner Solar System. The dramatic discovery of the cometary
nature of P/2021 HS highlights the challenges of detecting comets with
extremely low activity levels. Observations at high phase angle where forward
scattering is pronounced will help identify such comets.
