  We report the serendipitous discovery of transient, point-like high energy
gamma-ray emission coincident with the position of the suspected BL Lac object
5C 3.178. The source was detected using the Fermi Large Area Telescope (LAT) at
a significance level of ~8 sigma during a 200 day period which began on MJD
55882. The observed gamma-ray emission is characterised by a moderate 0.2-300
GeV flux F(0.2-300 GeV)=$(8.22\pm2.04)\times10^{-9}$ ph cm$^{-2}$s$^{-1}$ and a
hard power law spectrum with spectral index $\Gamma=1.76\pm0.09$. These
properties appear consistent with the known sub-population of TeV
gamma-ray-emitting BL Lac objects, implying that the source may be detectable
using atmospheric Cherenkov telescope arrays. Moreover, the temporal
variability of the source suggests that real-time searches of the Fermi-LAT
all-sky dataset for weak emission on ~200 day timescales may represent a
rewarding strategy.
