  In a dense colloidal suspension at a volume fraction slightly lower than that
of its glass transition, we follow the trajectories of an assembly of tracers
over a large time window. We define a local activity, which quantifies the
local tendency of the system to rearrange. We determine the statistics of the
time and space integrated activity, and we argue that it develops a low
activity tail that comes on a par with the onset of glassy behavior and
heterogeneous dynamics. These rare events may be interpreted as the reflection
of an underlying dynamic phase transition.
