We investigate the superconducting properties of Sc5Co4Si10 using low-temperature resistivity, magnetization, heat capacity, and muon-spin rotation and relaxation (μSR) measurements. We find that Sc5Co4Si10 exhibits type-II superconductivity with a superconducting transition temperature TC=3.5(1)K. The temperature dependence of the superfluid density obtained from transverse-field μSR spectra is best modeled using an isotropic Bardeen-Cooper-Schrieffer type s-wave gap symmetry with 2Δ/kBTC=2.84(2). However, the zero-field muon-spin relaxation rate reveals the appearance of a spontaneous magnetic field below TC, indicating that time-reversal symmetry (TRS) is broken in the superconducting state. Although this behavior is commonly associated with nonunitary or mixed singlet-triplet pairing, our group-theoretical analysis of the Ginzburg-Landau free energy alongside density functional theory calculations indicates that unconventional mechanisms are pretty unlikely. Therefore, we have hypothesized that TRS breaking may occur via a conventional electron-phonon process.