  Due to the small amount of hydrogen (${\leq 0.1M_{\odot}}$) remaining on the
surface of their progenitors, Type IIb supernovae are sensitive probes of the
mass loss processes of massive stars towards the ends of their lives, including
the role of binarity. We report late-time Hubble Space Telescope observations
of SN 2011dh in M51, and a brief period of re-brightening and plateau in the
photometric light curve, from $1.8$ to $6.2$ years after the explosion. These
observations exclude the role of circumstellar interaction, however a slow
rotating magnetar, a significant quantity of radioactive elements or a light
echo could be responsible for the late-time luminosity observed at $t >
1000\mathrm{d}$. If the late-time light curve is powered by the decay of
radioactive elements, SN~2011dh is required to have produced $\sim 2.6 \times
10^{-3}\,M_{\odot}$ of $\mathrm{^{44}Ti}$, which is significantly in excess of
the amount inferred from earlier nebular spectra of SN 2011dh itself or
measured in the Cas A SN remnant. The evolution of the brightness and the
colour of the late-time light curve also supports the role of a light echo
originating from dust with a preferred geometry of a disk of extent $\sim 1.8$
to $\sim 2.7\,\mathrm{pc}$ from the SN, consistent with a wind-blown bubble.
Accounting for the long term photometric evolution due to a light echo, the
flux contribution from a surviving binary companion at ultraviolet wavelengths
can be isolated and corresponds to a star of $\sim 9 - 10M_{\odot}$.
