  Short gamma-ray bursts (SGRBs) are now known to be the product of the merger
of two compact objects. However, two possible formation channels exist: neutron
star -- neutron star (NS -- NS) or NS -- black hole (BH). The landmark SGRB
170817A provided evidence for the NS -- NS channel, thanks to analysis of its
gravitational wave signal. We investigate the complete population of SGRBs with
an associated redshift (39 events), and search for any divisions that may
indicate that a NS -- BH formation channel also contributes. Though no
conclusive dichotomy is found, we find several lines of evidence that
tentatively support the hypothesis that SGRBs with extended emission (EE; 7
events) constitute the missing merger population: they are unique in the large
energy band-sensitivity of their durations, and have statistically distinct
energies and host galaxy offsets when compared to regular (non-EE) SGRBs. If
this is borne out via future gravitational wave detections it will conclusively
disprove the magnetar model for SGRBs. Furthermore, we identify the first
statistically significant anti-correlation between the offsets of SGRBs from
their host galaxies and their prompt emission energies.
