  A star wandering too close to a supermassive black hole (SMBH) will be
tidally disrupted. Previous studies of such "tidal disruption event" (TDE)
mostly focus on the stellar debris that are bound to the system, because they
give rise to luminous flares. On the other hand, half of the stellar debris in
principle are unbound and can stream to a great distance, but so far there is
no clear evidence that this "unbound debris stream" (UDS) exists. Motivated by
the fact that the circum-nuclear region around SMBHs is usually filled with
dense molecular clouds (MCs), here we investigate the observational signatures
resulting from the collision between an UDS and a MC, which is likely to happen
hundreds of years after a TDE. We focus on $\gamma$-ray emission ($0.1-10^5$
GeV), which comes from the encounter of shock-accelerated cosmic rays with
background protons and, more importantly, is not subject to extinction. We show
that because of the high proton density inside a MC, the peak $\gamma$-ray
luminosity, about $10^{39}~{\rm erg~s^{-1}}$, is at least $100$ times greater
than that in the case without a MC (only with a smooth interstellar medium).
The luminosity decays on a time-scale of decades, depending on the distance of
the MC, and about a dozen of these "TDE afterglows" could be detected within a
distance of about $16$ Mpc by the future Cherenkov Telescope Array. Without
careful discrimination, these sources potentially could contaminate the
searches for starburt galaxies, galactic nuclei containing millisecond pulsars
or dark-matter annihilation signals.
