The rate of "fast" optical transients (defined as sources that fade by more than 1 magnitude within an hour), in several surveys searching for orphan afterglow emission.  The volumetric rate of such events is entirely consistent with the rate of on-axis GRB optical afterglows, bounding the rate of "orphan" events from off-axis or "cloaked" fireballs.  From \cite{Cenko2015}.