  The Na D absorption doublet in the spectrum of $\eta$ Carinae is complex,
with multiple absorption features associated with the Great Eruption (1840s),
the Lesser Eruption (1890s), and interstellar clouds. The velocity profile is
further complicated by the P Cygni profile originating in the system's stellar
winds and blending with the He I $\lambda$5876 profile. The Na D profile
contains a multitude of absorption components, including those at velocities of
$-$145 km s$^{-1}$, $-$168 km s$^{-1}$, and $+$87 km s$^{-1}$ that we
concentrate on in this analysis. Ground-based spectra recorded from 2008 to
2021 show significant variability of the $-$145 km s$^{-1}$ absorption
throughout long-term observations. In the high ionization phases of $\eta$
Carinae prior to the 2020 periastron passage, this feature disappeared
completely but briefly reappeared across the 2020 periastron, along with a
second absorption at $-$168 km s$^{-1}$. Over the past few decades, $\eta$ Car
has been gradually brightening demonstrated to be caused by a dissipating
occulter. The decreasing absorption of the $-$145 km s$^{-1}$ component,
coupled with similar trends seen in absorptions of ultraviolet resonant lines,
indicate that this central occulter was possibly a large clump associated with
the Little Homunculus or another clump between the Little Homunculus and the
star. We also report on a foreground absorption component at $+$87 km s$^{-1}$.
Comparison of Na D absorption in the spectra of nearby systems demonstrates
that this red-shifted component likely originates in an extended foreground
structure consistent with a previous ultraviolet spectral survey in the Carina
Nebula.
