  Within the standard model, non-renormalizable operators at dimension six
($d=6$) violate baryon and lepton number by one unit and thus lead to proton
decay. Here, we point out that the proton decay mode with a charged pion and
missing energy can be a characteristic signature of $d=6$ operators containing
a light sterile neutrino, if it is not accompanied by the standard $\pi^0e^+$
final state. We discuss this effect first at the level of effective operators
and then provide a concrete model with new physics at the TeV scale, in which
the lightness of the active neutrinos and the stability of the proton are
related.
