The mechanism of hydrogen spillover has long been explained by a chemical kinetics analysis, which leads to a P^0.5 pressure dependence of the spillover process, suggesting the uptake will be appreciably increased as pressures are extrapolated to those of interest for DOE hydrogen storage targets (i.e. 100 bar). However, using a high-pressure gravimetric balance, we have experimentally observed mass uptake that can only be attributed to hydrogen spillover at extremely low pressures (i.e. 10-7 bar), inconsistent with models that predict a P^0.5 pressure dependence. The uptake is observed for samples containing metal and carbon, and not observed for inert samples or carbon samples without metal. Our results also indicate that pretreatment is a key parameter to observe the low pressure uptake. The results have important implications as to how overall uptake is calculated in gravimetric versus volumetric adsorption measurements: for the latter case, the low-pressure spillover is not detectable. Mechanistic implications of hydrogen spillover for hydrogen storage will be discussed.