Microlensing by brown dwarfs: the case of dark clusters
  Experiments are looking for diffuse brown dwarfs in the dark galactic halo through the gravitational lens effect. If brown dwarfs are clumped in dark clusters, the event rate is not changed, but events are spatially clustered, and stars nearby a micro-lensed one are likely to be micro-lensed in a near future. Therefore, an intensive survey of the region where a micro-lensing occurred should reveal many other events. 
Alain Bouquet, Jean Kaplan and Jean-Manuel Verschaeve