  Recently the Event Horizon Telescope observed black holes at event horizon
scales for the first time, enabling us to now test the existence of event
horizons. Although event horizons have by definition no observable features,
one can look for their non-existence. In that case, it is likely that there is
some kind of surface, which like any other surface could absorb (and thermally
emit) and/or reflect radiation. In this paper, we study the potential
observable features of such rotating reflecting surfaces. We construct a
general description of reflecting surfaces in arbitrary spacetimes. This is
used to define specific models for static and rotating reflecting surfaces, of
which we study the corresponding light paths and synthetic images. This is done
by numerical integration of the geodesic equation and by the use of the general
relativistic radiative transfer code RAPTOR. The reflecting surface creates an
infinite set of ring-like features in synthetic images inside the photon ring.
There is a central ring in the middle and higher order rings subsequently lie
exterior to each other converging to the photon ring. The shape and size of the
ring features change only slightly with the radius of the surface R, spin a and
inclination i, resulting in all cases in features inside the 'shadow region'.
We conclude that rotating reflecting surfaces have clear observable features
and that the Event Horizon Telescope is able to observe the difference between
reflecting surfaces and an event horizon for high reflectivities. Such
reflecting surface models can be excluded, which strengthens the conclusion
that the black hole shadow indeed indicates the existence of an event horizon.
