  In this paper we present multiband optical polarimetric observations of the
VHE blazar PKS 2155-304 made simultaneously with a H.E.S.S./Fermi high-energy
campaign in 2008, when the source was found to be in a low state. The intense
daily coverage of the dataset allowed us to study in detail the temporal
evolution of the emission and we found that the particle acceleration
timescales are decoupled from the changes in the polarimetric properties of the
source. We present a model in which the optical polarimetric emission
originates at the polarised mm-wave core and propose an explanation for the
lack of correlation between the photometric and polarimetric fluxes. The
optical emission is consistent with an inhomogeneous synchrotron source in
which the large scale field is locally organised by a shock in which particle
acceleration takes place. Finally, we use these optical polarimetric
observations of PKS 2155-304 at a low state to propose an origin for the
quiescent gamma-ray flux of the object, in an attempt to provide clues for the
source of its recently established persistent TeV emission.
