  Bogoliubov Fermi surfaces (BFSs) are topologically protected regions of zero
energy excitations in a superconductor whose dimension equals that of the
underlying normal state Fermi surface. Examples of Hamiltonians exhibiting this
"ultranodal" phase are known to preserve charge-conjugation ($C$) and parity
($P$) but break time-reversal ($T$). In this work, we provide examples of model
Hamiltonians that do not necessarily preserve this symmetry pattern but have
well-defined sign-changing Pfaffians yielding BFSs. While their topological
character has not been recognized previously, some of the models we present
have been extensively studied in prior literature. We further examine
thermodynamic and electronic properties arising from the ultranodal state. In
particular, we study the effect of a weak Zeeman field close to the topological
transition and propose distinguishing features of BFSs using residual specific
heat and tunneling conductance. Our calculation of the superfluid density in a
toy multi-band model indicates a window of interband pairing strength where
BFSs are stable with a positive superfluid density. We also present additional
signatures of BFSs in spin-polarized spectral weight and total magnetization
measurements.
