  The predicted interplay between Kondo physics and non-trivial topology in
SmB$_{6}$ has stimulated many experimental reports, some of which are in
apparent contradiction. The origin of the dispute may lie on the fragility of
the Kondo insulating phase in the presence of Sm vacancies (Kondo holes) and/or
natural impurities, such as Gd$^{3+}$. In this work, we locally investigated
this fragility for Al-flux grown Sm$_{1-x}$Gd$_{x}$B$_{6}$ single crystals (0
$\leq$ $x$ $\leq$ 0.02) by combining electron spin resonance (ESR) and
complementary bulk measurements. The Gd$^{3+}$ ESR spectra in a highly dilute
regime ($x$ $\sim 0.0004$) display the features of an insulating cubic
environment. Remarkably, a metallic ESR lineshape is observed for more
concentrated samples ($x$ $\geq$ 0.004), even though these systems are still in
a reasonably dilute regime and show insulating $dc$ electrical resistivity. Our
data indicate that the Kondo insulating state is destroyed locally around
impurities before a global percolation occurs. This result not only explains
the discrepancy between $dc$ and $ac$ conductivity, but also provides a
scenario to explain the presence of quantum oscillations in magnetization in
the absence of quantum oscillations in electrical resistivity.
