  We examine how the (2+1)-dimensional AdS space is covered by the
Fefferman-Graham system of coordinates for Minkowski, Rindler and static de
Sitter boundary metrics. We find that, in the last two cases, the coordinates
do not cover the full AdS space. On a constant-time slice, the line delimiting
the excluded region has endpoints at the locations of the horizons of the
boundary metric. Its length, after an appropriate regularization, reproduces
the entropy of the dual CFT on the boundary background. The horizon can be
interpreted as the holographic image of the line segment delimiting the
excluded region in the vicinity of the boundary.
