  With the advent of gravitational wave astronomy, searching for gravitational
wave echoes from black holes (BHs) is becoming an interesting probe of their
quantum nature near their horizons. Newborn BHs may be strong emitters of
echoes, as they accompany large perturbations in the surrounding spacetime upon
formation. Utilizing the Quantum Black Hole Seismology framework (Oshita et al.
2020), we study the expected echoes upon BH formation resulting from neutron
star mergers and failed supernovae. For BH remnants from neutron star mergers,
we evaluate the consistency of these models with the recent claim on the
existence of echoes following the neutron star merger event GW170817. We find
that the claimed echoes in GW170817, if real, suggest that overtones contribute
a significant amount of energy in the ringdown of the remnant BH. We finally
discuss the detectability of echoes from failed supernovae by second and
third-generation gravitational wave detectors, and find that current (future)
detectors constrain physical reflectivity models for events occurring within a
few Mpc (a few x 10 Mpc). Detecting such echo signals may significantly
constrain the maximum mass and equation of state of neutron stars.
