  In the case where the Standard Model is extended by one heavy Majorana
fermion, the branching fractions of semileptonic meson decays into same-sign
and opposite-sign dileptons are expected to be of the same order. As we discuss
here, this need not be the case in extensions by at least two sterile fermions,
due to the possible destructive and constructive interferences that might
arise. Depending on the $CP$ violating phases, one can have an enhancement of
the lepton number violating modes and suppression of the lepton number
conserving ones (and vice-versa). We explore for the first time the
interference effects in semileptonic decays, and illustrate them for a future
observation of kaon decays at NA62. We also argue that a non-observation of a
given mode need not be interpreted in terms of reduced active-sterile mixings,
but that it could instead be understood in terms of interference effects due to
the presence of several sterile states; in particular, for different-flavour
final state charged leptons, observing a lepton number conserving process and
not a lepton number violating one does not rule out that the mediators are
Majorana fermions.
