  Self-propelled colloids (swimmers) in confining geometries follow
trajectories determined by hydrodynamic interactions with the bounding
surfaces. However, typically these interactions are ignored or truncated to
lowest order. We demonstrate that higher-order hydrodynamic moments cause
rod-like swimmers to follow oscillatory trajectories in quiescent fluid between
two parallel plates, using a combination of lattice-Boltzmann simulations and
far-field calculations. This behavior occurs even far from the confining walls
and does not require lubrication results. We show that a swimmer's hydrodynamic
quadrupole moment is crucial to the onset of the oscillatory trajectories. This
insight allows us to develop a simple model for the dynamics near the channel
center based on these higher hydrodynamic moments, and suggests opportunities
for trajectory-based experimental characterization of swimmers' hydrodynamic
properties.
