[en] String Theory and Supergravity allow, in principle, to follow the transition of the inflaton from pre-inflationary fast roll to slow roll. This introduces an infrared depression in the primordial power spectrum that might have left an imprint in the CMB anisotropy, if it occurred at accessible wavelengths. We model the effect extending $\Lambda \mathrm{CDM}$ with a scale $\Delta$ related to the infrared depression and explore the constraints allowed by Planck 2015 data, employing also more conservative, wider Galactic masks in the low resolution CMB likelihood. In an extended mask with $f_{sky}=39\%$, we thus find $\Delta =(0.351\pm 0.114)\times 10^{-3}{\text{Mpc}}^{-1}$, at 99.4% confidence level, to be compared with a nearby value at 88.5% with the standard $f_{sky}=94\%$ mask. With about 64 $e$-folds of inflation, these values for $\Delta$ would translate into primordial energy scales $\mathcal{O}\left({10}^{14}\right)\mathrm{GeV}$.