  FeSe$_{1-x}$S$_x$ remains one of the most enigmatic systems of Fe-based
superconductors. While much is known about the orthorhombic parent compound,
FeSe, the tetragonal samples, FeSe$_{1-x}$S$_x$ with x>0.17, remain relatively
unexplored. Here, we provide an in-depth investigation of the electronic states
of tetragonal FeSe$_{0.81}$S$_{0.19}$, using scanning tunneling microscopy and
spectroscopy (STM/S) measurements, supported by angle-resolved photoemission
spectroscopy (ARPES) and theoretical modeling. We demonstrate that by analyzing
modulations of the local density of states (LDOS) near and away from Fe vacancy
defects separately, we can identify quasiparticle interference (QPI) signals
originating from multiple regions of the Brillouin zone, including the bands at
the M and A points. We also observe that QPI signals coexist with a much
stronger LDOS modulation for states near the Fermi level whose period is
independent of energy. Our measurements further reveal that this strong pattern
appears in the STS measurements as short range stripe patterns that are locally
two-fold symmetric. Since these stripe patterns coexist with four-fold
symmetric QPI around Fe-vacancies, the origin of their local two-fold symmetry
must be distinct from that of nematic states in orthorhombic samples. To
further understand these stripe patterns, we explore several aspects related to
them, such as the role of S and Fe vacancy defects, and whether they can be
explained by QPI. We consider the possibility that the observed stripe patterns
may represent incipient charge order correlations, similar to those observed in
the cuprates.
