  We consider guarding classes of simple polygons using mobile guards (polygon
edges and diagonals) under the constraint that no two guards may see each
other. In contrast to most other art gallery problems, existence is the primary
question: does a specific type of polygon admit some guard set? Types include
simple polygons and the subclasses of orthogonal, monotone, and starshaped
polygons. Additionally, guards may either exclude or include the endpoints
(so-called open and closed guards). We provide a nearly complete set of answers
to existence questions of open and closed edge, diagonal, and mobile guards in
simple, orthogonal, monotone, and starshaped polygons, with some surprising
results. For instance, every monotone or starshaped polygon can be guarded
using hidden open mobile (edge or diagonal) guards, but not necessarily with
hidden open edge or hidden open diagonal guards.
