  We investigate the finite-temperature properties of the $S$=1/2
Shastry-Sutherland Heisenberg model using a quantum typicality method. In the
intermediate plaquette state region, we naturally expect to realize the
finite-temperature phase transition associated with breaking the mirror
symmetry of this model. We reveal some signatures of the spontaneous phase
transition within a two-point correlation level at moderate temperatures since
the constructed typical state can sense the existence of the degenerated
excited states depending on the initial random state. We also confirm that the
local mirror order parameter shows the intriguing recovering phenomenon of the
mirror symmetry in very low-temperatures, which could be understood from the
nature of the ground and excited states of the finite-size systems. We expect
that this recovering feature disappears; instead, a saturated behavior appears
in the local mirror order parameter in the thermodynamic limit. We discuss the
relationship to the recent experimental results on ${\rm SrCu_2(BO_3)_2}$ under
high pressures.
