  Comet 2I/Borisov, the first unambiguous interstellar comet ever found, was
discovered in August 2019 at $\sim3$ au from the Sun on its inbound leg. No
pre-discovery detection beyond 3 au has yet been reported, mostly due to the
comet's proximity to the Sun as seen from the Earth. Here we present a search
for pre-discovery detections of comet Borisov using images taken by the
Catalina Sky Survey (CSS), Pan-STARRS and Zwicky Transient Facility (ZTF), with
a further comprehensive follow-up campaign being presented in
\citet{Bolin2019}. We identified comet Borisov in ZTF images taken in May 2019
and use these data to update its orbit. This allowed us to identify the comet
in images acquired as far back as December 2018, when it was 7.8 au from the
Sun. The comet was not detected in November 2018 when it was 8.6 au from the
Sun, possibly implying an onset of activity around this time. This suggests
that the activity of the comet is either driven by a more volatile species
other than H$_2$O, such as CO or CO$_2$, or by exothermic crystallization of
amorphous ice. We derive the radius of the nucleus to be $<7$ km using the
non-detection in November 2018, and estimate an area of $\sim0.5$---$10
\mathrm{km^2}$ has been active between December 2018 and September 2019, though
this number is model-dependent and is highly uncertain. The behavior of comet
Borisov during its inbound leg is observationally consistent with dynamically
new comets observed in our solar system, suggesting some similarities between
the two.
