  We present evidence of variable 1.3 millimeter emission from the 1-3 Myr, SpT
G2-G5 class III YSO, HD~283572. HD~283572 was observed on 8 dates with the
Submillimeter Array between 2021 December and 2023 May, a total on-source time
of 10.2 hours, probing a range of timescales down to 5.2 seconds. Averaging all
data obtained on 2022 Jan 17 shows a 4.4 mJy ($8.8\sigma$) point source
detection with a negative spectral index ($\alpha{=}{-2.7}{\pm}1.2$), with peak
emission rising to 13.8 mJy in one 3 minute span, and 25 mJy in one 29.7 second
integration ($L_\nu=4.7\times10^{17}$ erg s$^{-1}$ Hz$^{-1}$). Combining our
data for the other 7 dates shows no detection, with an rms noise of 0.24 mJy
beam$^{-1}$. The stochastic millimeter enhancements on time frames of
seconds--minutes--hours with negative spectral indices are most plausibly
explained by synchrotron or gyro-synchrotron radiation from stellar activity.
HD 283572's 1.3 mm light-curve has similarities with variable binaries,
suggesting HD 283572's activity may have been triggered by interactions with an
as-yet undetected companion. We additionally identify variability of HD 283572
at 10 cm, from VLASS data. This study highlights the challenges of interpreting
faint mm emission from evolved YSOs that may host tenuous disks, and suggests
that a more detailed temporal analysis of spatially unresolved data is
generally warranted. The variability of class III stars may open up new ground
for understanding the physics of flares in the context of terrestrial planet
formation.
