  We report helium absorption from the escaping atmosphere of TOI 560.01 (HD
73583b), a $R=2.8 R_\oplus$, $P=6.4$ d mini Neptune orbiting a young ($\sim$600
Myr) K dwarf. Using Keck/NIRSPEC, we detect a signal with an average depth of
$0.68 \pm 0.08$% in the line core. The absorption signal repeats during a
partial transit obtained a month later, but is marginally stronger and bluer,
perhaps reflecting changes in the stellar wind environment. Ingress occurs on
time, and egress occurs within 12 minutes of the white light egress, although
absorption rises more gradually than it declines. This suggests that the
outflow is slightly asymmetric and confined to regions close to the planet. The
absorption signal also exhibits a slight 4 km/s redshift rather than the
expected blueshift; this might be explained if the planet has a modest orbital
eccentricity, although the radial velocity data disfavors such an explanation.
We use XMM-Newton observations to reconstruct the high energy stellar spectrum
and model the planet's outflow with 1D and 3D hydrodynamic simulations. We find
that our models generally overpredict the measured magnitude of the absorption
during transit, the size of the blueshift, or both. Increasing the metallicity
to 100$\times$ solar suppresses the signal, but the dependence of the predicted
signal strength on metallicity is non-monotonic. Decreasing the assumed stellar
EUV flux by a factor of 3 likewise suppresses the signal substantially.
