  The recent measurements of Cosmic Microwave Background temperature and
polarization anisotropies made by the Planck satellite have provided impressive
confirmation of the $\Lambda$CDM cosmological model. However interesting hints
of slight deviations from $\Lambda$CDM have been found, including a $95 \%$
c.l. preference for a "modified gravity" structure formation scenario. In this
paper we confirm the preference for a modified gravity scenario from Planck
2015 data, find that modified gravity solves the so-called $A_{lens}$ anomaly
in the CMB angular spectrum, and constrains the amplitude of matter density
fluctuations to $\sigma_8=0.815_{-0.048}^{+0.032}$, in better agreement with
weak lensing constraints. Moreover, we find a lower value for the reionization
optical depth of $\tau=0.059\pm0.020$ (to be compared with the value of $\tau=
0.079 \pm 0.017$ obtained in the standard scenario), more consistent with
recent optical and UV data. We check the stability of this result by
considering possible degeneracies with other parameters, including the neutrino
effective number, the running of the spectral index and the amount of
primordial helium. The indication for modified gravity is still present at
about $95\%$ c.l., and could become more significant if lower values of $\tau$
were to be further confirmed by future cosmological and astrophysical data.
