Silicon photomultipliers (SiPMs) from various manufacturers are shown to be sensitive to gamma-ray ionizing radiation without being coupled to a scintillator. Survey measurements of units from Hamamatsu, SENSL, and KETEK are shown, followed by detailed measurements of units from SENSL. Pulseheight spectroscopy of gamma-ray-induced signals reveals large pulses in excess of 16 microcells excited per event and as large as ~100 excited microcells. We review other examples of direct excitation by ionizing radiation found in the literature and surmise the effect that is related to the scattered charged particles (electrons) passing through the SiPM. We demonstrate that this direct sensitivity extends to cosmic-ray secondaries by a triple-coincidence experiment. The precise signal producing mechanism is unclear. The signal may be due to light producing processes, such as Čerenkov or scintillation in the optical windows, or charged particle ionization in the microcell layer, which may induce avalanches in microcells along the particle track. Possible applications and avenues for future study are suggested.