  The sign and profile of the thermal Hall conductivity $\kappa_{xy}$ gives
important insights into the gap structure of multi-band superconductors. With
this perspective, we have investigated $\kappa_{xy}$ and the thermal
conductivity $\kappa_{xx}$ in $\rm Ba_{1-x}K_xFe_2As_2$ which display large
peak anomalies in the superconducting state. The anomalies imply that a large
hole-like quasiparticle (qp) population exists below the critical temperature
$T_c$. We show that the qp mean-free-path inferred from $\kappa_{xx}$
reproduces the observed anomaly in $\kappa_{xy}$, providing a consistent
estimate of a large qp population. Further, we demonstrate that the hole-like
signal is consistent with a theoretical scenario where despite potentially
large gap variations on the electron pockets, the minimal homogeneous gap of
the superconducting phase resides at a hole pocket. Implications for probing
the gap structure in the broader class of pnictide superconductors are
discussed.
