Most untapped high-enthalpy geothermal resources are blind, meaning lacking surface evidence of their existence. The first step in their discovery is to find evidence of hydrothermal activity. Here we apply an approach based on the geochemistry of cold waters, which allowed us to identify evidence of the existence of a hydrothermal system at Piton de la Fournaise volcano (Réunion Island), and constrain its location. This approach uses the concentrations in B, Li, SO4, F, Mo, P, V, As and HCO3 and the isotopic ratios δ13C and δ11B as geochemical markers of hydrothermal activity that can be used even in waters with extremely low ion content (Electrical conductivity <80 µS/cm), and even when their geochemical composition is mainly controlled by other processes. This noninvasive approach is easy to implement and can be applied wherever the presence of a blind geothermal system is suspected.