We analyzed the surfaces of hydrogen-intercalated graphene on SiC(0001) by photoelectron spectroscopy and Raman spectroscopy after applying electric currents to the graphene for carrier mobility measurements while the sample was exposed to air. We found that Si at the interface between graphene and SiC oxidizes very gradually, whereas the graphene does not. The Si of this Si oxide is mainly divalent, and the oxide is attributed to SiO molecules. These molecules are volatile, and they have not been observed either on the oxidized surface of SiC(0001) or on oxygen-intercalated graphene/SiC(0001) formed by annealing in air or oxygen. This anomalous oxidation, which is triggered by the application of current to the graphene overlayer, could be observed because of the presence of the graphene overlayer and the oxidation at room temperature.