We experimentally investigate the detection mechanism in a meandered molybdenum silicide superconducting nanowire single-photon detector by characterizing the detection probability as a function of bias current in the wavelength range of 750–2050 nm. Contrary to some previous observations on niobium nitride or tungsten silicide detectors, we find that the energy-current relation is nonlinear in this range. Furthermore, thanks to the presence of a saturated detection efficiency over the whole range of wavelengths, we precisely quantify the shape of the curves. This allows a detailed study of their features, which are indicative of both Fano fluctuations and position-dependent effects.