  London and Campbell penetration depths were measured in single crystals of
the endohedral gallide cluster superconductor, Mo$_{8}$Ga$_{41}$. The full
temperature range superfluid density is consistent with the clean isotropic
$s-$wave weak-coupling BCS theory without any signs of the second gap or strong
coupling. The temperature dependence of the Campbell length is hysteretic
between zero-field cooling (ZFC) and field-cooling (FC) protocols, indicating
an anharmonic vortex pinning potential. The field dependence of the effective
critical current density, $j_{c}\left(H\right)$, reveals an unusual result.
While in the ZFC protocol, $j_{c}\left(H\right)$ is monotonically suppressed by
the magnetic field, it exhibits a profound ``hidden'' peak effect in the FC
protocol, that is, without a vortex density gradient. We suggest a possible
novel mechanism for the formation of the peak effect, which involves both
static and dynamic aspects.
