  While the impurity-induced nanoscale electronic disorder has been extensively
reported in the underdoped iron pnictides, its microscopic origins remain
elusive. Recent scanning tunneling microscopy (STM) measurements reveal a
dimer-type resonant structure induced by cobalt doping. These dimers are
randomly distributed but uniformly aligned with the antiferromagnetic a axis. A
theory of the impurity-induced quasiparticle interference patterns is presented
that shows the local density of states developing an oscillatory pattern
characterized by both geometry and orbital content of the {\em reconstructed}
Fermi pockets, occasioned by the pocket density-wave (PoDW) order along the b
axis. This pattern breaks the $C_4$ symmetry and its size and orientation
compare well with the dimer resonances found in the STM experiments, hinting at
the presence of a "hidden" PoDW order. More broadly, our theory spotlights such
nanoscale structures as a useful diagnostic tool for various forms of order in
iron pnictides.
