  The validity of the fluctuation theorem for entropy production as deduced
from the observation of trajectories implicitly requires that all slow degrees
of freedom are accessible. We experimentally investigate the role of hidden
slow degrees of freedom in a system of two magnetically coupled driven
colloidal particles. The apparent entropy production based on the observation
of just one particle obeys a fluctuation theorem-like symmetry with a slope of
1 in the short time limit. For longer times, we find a constant slope, but
different from 1. We present theoretical arguments for a generic linear
behavior both for small and large apparent entropy production but not
necessarily throughout. By fine-tuning experimental parameters, such an
intermediate nonlinear behavior can indeed be recovered in our system as well.
