  We continue investigation of the hidden plane-mirror symmetry in the
distribution of excursion sets in cosmic microwave background (CMB) temperature
anisotropy maps, previously noticed in the three-year data of the Wilkinson
microwave anisotropy probe (WMAP), using the WMAP 5 years maps. The symmetry is
shown to have higher significance, \chi^2 < 1.7, for low multipoles \ell < 5,
while disappearing at larger multipoles, \chi^2 > 3.5 for \ell > 10. The study
of the sum and difference maps of temperature inhomogeneity regions, along with
simulated maps, confirms its existence.The properties of these mirroring
symmetries are compatible with those produced by the Sachs-Wolfe effect in the
presence of an anomalously large component of horizon-size density
perturbations, independent of one of the spatial coordinates, and/or a
slab-like spatial topology of the Universe.
