There's a side culvert coming into the main tunnel, near where Eva is standing.  Freki can tell the stench when he approaches Eva.  Eva and Meeps catch the scent long after Freki, and wonder why he's sniffing, until they smell it.  When you investigate, there's a corpse within.  It's obviously a dead guy.  You see the light from a manhole about 20 feet behind him, he must have fallen in.  But why didn't you smell him the first time you passed?