I made a minimal scene which reproduces the phenomenon.

  Comment out the box, and the caustic highlight appears. With the box there's
no caustic. If the box has a transmit value smaller than 1, the caustic appears
on the top surface of the box but doesn't go through.

  I'm not sure why this happens.

//-------------------------------------------------------------------------
global_settings { photons { count 10000 } }
camera { location <0, 1, -5> look_at -y*.5 }
light_source { y*10, 1 }
plane { y, -2 pigment { rgb 1 } }
sphere
{ 0, 1
  pigment { rgb <.9, 1, .9> filter 1 }
  finish { specular .5 roughness .01 reflection { .1, .3 } }
  interior { ior 1.5 }
  photons { target reflection on refraction on }
}

box
{ <-2, -1.9, -2>, <2, -1.5, 2>
  pigment { rgbt 1 }
  photons { pass_through }
}
//-------------------------------------------------------------------------

-- 
                                                          - Warp

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