We develop an unbiased intermittency analysis scheme based on the AMIAS \cite{AMIAS} protocol. We apply it to analyse fluctuations in transverse momentum space of protons produced in ion collisions at CERN SPS experiments NA49 and NA61/SHINE, our main goal being the unbiased determination of the intermittency index $\phi_2$ \cite{NGA1}. We find a gradual enhancement of $\phi_2$ as the production source of the considered ensemble of events changes from central to peripheral Ar+Sc collisions at $\sqrt{s_{NN}}=16.8$GeV (NA61/SHINE) and subsequently to central Si+Si collisions at $\sqrt{s_{NN}}=17.3$GeV (NA49). A possible explanation of these results is the gradual proximity to the QCD critical endpoint, remnant of the chiral transition, which should be located in the immediate neighbourhood of the Si+Si freeze-out state.