  Odd-frequency pairing is an unconventional type of Cooper pairing in
superconductors related to the frequency dependence of the corresponding
anomalous Green function. We show by a combination of analytical and numerical
methods that odd-frequency pairing is ubiquitously present in the current of
Andreev-scattered particles across a junction formed by a normal metal (N) and
a superconductor (S), even if the superconducting pairing is of conventional
$s$-wave, spin singlet type. We carefully analyze the conductance of NS
junctions with different pairing symmetries ($s$-wave, $p$-wave, $d$-wave). In
all cases, we identify a generic equal balance of even and odd frequency
pairing to the contributions related to Andreev reflection. This analysis shows
in retrospect that the presence of odd-frequency pairing in electric currents
across NS junctions is rather the rule, not the exception. This insight stems
from an alternative approach of analyzing the transport problem of hybrid
structures. It is based on the Kubo-Greenwood formula with direct access to
symmetries of the anomalous Green functions characterizing the superconducting
pairing. We expect that our predictions substantially enrich the interpretation
of transport data across NS junctions in many material combinations.
