Tuning of the catalytic reaction rate by electric polarization of the interface between an electron conducting catalyst and an ion conducting support, called electrochemical promotion of catalysis (EPOC), is most often fully reversible. Its state-of-the-art model regards the gas-exposed catalyst surface as the unique location of charge storage via backspillover of electrochemically generated species, responsible for promotion. After long-lasting anodic polarization, a permanent effect (P-EPOC) was observed in ethylene combustion with oxygen over Pt/YSZ catalyst. Double step chronoamperometric and linear sweep voltammetric analysis revealed delayed oxygen storage, located presumably at the vicinity of the catalyst/electrolyte interface. It is proposed that oxygen stored at this location, hence hidden for the reactants and then. released during relaxation, was at the origin of the observed P-EPOC. The effect of this 'hidden' promoter on the catalytic reaction rate was found to be highly non Faradaic.