  We study U-DHOST theories, i.e., higher-order scalar-tensor theories which
are degenerate only in the unitary gauge and yield an apparently unstable extra
mode in a generic coordinate system. We show that the extra mode satisfies a
three-dimensional elliptic differential equation on a spacelike hypersurface,
and hence it does not propagate. We clarify how to treat this "shadowy" mode at
both the linear and the nonlinear levels.
