  The optical conductivity of Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ shows a
clear signature of the superconducting gap, but a simple $s$-wave description
fails in accounting for the low frequency response. This task is achieved by
introducing an extra Drude peak in the superconducting state representing
sub-gap absorption, other than thermally broken pairs. This extra peak and the
coexisting $s$-wave response respect the total sum rule indicating a common
origin for the carriers. We discuss the possible origins for this absorption as
(i) quasiparticles due to pair-breaking from interband impurity scattering in a
two band $s_{\pm}$ gap symmetry model, which includes (ii) the possible
existence of impurity levels within an isotropic gap model; or (iii) an
indication that one of the bands is highly anisotropic.
