We investigate the potential (small) deviations from the unitarity of the
mixing matrix that are expected to occur, because of mass splittings, in the
Quantum Field Theory of non-degenerate coupled systems. We extend our previous
analysis concerning mixing angles, which led to a precise determination of the
Cabibbo angle, to the case of three generations of fermions. We show that the
same condition for neutral currents of mass eigenstates, i.e. that universality
of diagonal currents is violated with the same strength as the absence of
non-diagonal ones, is satisfied: on one hand, by the three CKM mixing angles
with a precision higher than the experimental uncertainty; on the other hand,
by a neutrino-like mixing pattern in which theta_{23} is maximal, and tan (2
theta_{12})=2. This last pattern turns out to satisfy exactly the "quark-lepton
complementarity condition" theta_c + theta_{12}= pi/4. Moreover, among all
solutions, two values for the third neutrino mixing angle arise which satisfy
the bound sin^2(theta_{13}) < 0.1: theta_{13} = +/- 5.7 10^{-3} and theta_{13}
= +/- 0.2717. The so-called "Neighborhood of the Standard Model" is thus
confirmed to exhibit special patterns which presumably originate in physics
"Beyond the Standard Model"
