  Quasi-periodic moir\'{e} patterns and their effect on electronic properties
of twisted bilayer graphene (TBG) have been intensely studied. At small twist
angle $\theta$, due to atomic reconstruction, the moir\'e superlattice morphs
into a network of narrow domain walls separating micron-scale AB and BA
stacking regions. We use scanning probe photocurrent imaging to resolve
nanoscale variations of the Seebeck coefficient occurring at these domain
walls. The observed features become enhanced in a range of mid-infrared
frequencies where the hexagonal boron nitride (hBN), which we use as a TBG
substrate, is optically hyperbolic. Our results illustrate new capabilities of
nano-photocurrent technique for probing nanoscale electronic inhomogeneities in
two-dimensional materials.
