  A single static magnetic impurity in a fully-gapped superconductor leads to
formation of an intragap quasiparticle bound state. At temperatures much below
the superconducting transition, the energy relaxation and spin dephasing of the
state are expected to be exponentially suppressed. The presence of such a state
can be detected in electron tunneling experiments as a pair of conductance
peaks at positive and negative biases. Here we show, that for an arbitrarily
weak tunneling strength, the peaks have to be symmetric with respect to the
applied bias. This is in contrast to the standard result that the tunneling
conductance is proportional to the local (in general particle-hole asymmetric)
density of states. The asymmetry can be recovered is one allows for either a
finite density of impurity states, or that impurities are coupled to another,
non-superconducting, equilibrium bath.
