For most physics events and beam gas collisions particles hit the FCAL beam pipe region. The cells in this region usually form a single large condensate. If the traversing muon hits this condensate MUFFIN is unable to identify the muon because there are too many cells in the condensate which are not hit by the muon.
MUFFIN finds the muon in such events by removing successively rings of cells around the FCAL beam pipe hole and rerunning all of its algorithms.