  We present evidence that the recently discovered, directly-imaged planet HD
131399 Ab is a background star with non-zero proper motion. From new JHK1L'
photometry and spectroscopy obtained with the Gemini Planet Imager, VLT/SPHERE,
and Keck/NIRC2, and a reanalysis of the discovery data obtained with
VLT/SPHERE, we derive colors, spectra, and astrometry for HD 131399 Ab. The
broader wavelength coverage and higher data quality allow us to re-investigate
its status. Its near-infrared spectral energy distribution excludes spectral
types later than L0 and is consistent with a K or M dwarf, which are the most
likely candidates for a background object in this direction at the apparent
magnitude observed. If it were a physically associated object, the projected
velocity of HD 131399 Ab would exceed escape velocity given the mass and
distance to HD 131399 A. We show that HD 131399 Ab is also not following the
expected track for a stationary background star at infinite distance. Solving
for the proper motion and parallax required to explain the relative motion of
HD 131399 Ab, we find a proper motion of 12.3 mas/yr. When compared to
predicted background objects drawn from a galactic model, we find this proper
motion to be high, but consistent with the top 4% fastest-moving background
stars. From our analysis we conclude that HD 131399 Ab is a background K or M
dwarf.
