  We characterize a high-density sample of negatively charged silicon-vacancy
(SiV$^-$) centers in diamond using collinear optical multidimensional coherent
spectroscopy. By comparing the results of complementary signal detection
schemes, we identify a hidden population of \ce{SiV^-} centers that is not
typically observed in photoluminescence, and which exhibits significant
spectral inhomogeneity and extended electronic $T_2$ times. The phenomenon is
likely caused by strain, indicating a potential mechanism for controlling
electric coherence in color-center-based quantum devices.
