Osmosis is variously described as the passage of a solution or solvent from one area of high concentration to another of low concentration through a semi-permeable membrane. The basic concept is appliable to the work situation as well. Someone leaves through osmosis someone else takes over their job (the area of low concentration). This often occurs in companies under severe budgetary constraints, someone else takes on a role as an official or unofficial secondment. This is followed by one of two things. Either osmosis takes them back to their original role as that becomes under-concentrated or another body fills that as they realise it is under-concentrated.
That actually made more sense when it occurred to me in that brief moment of lucidity that precedes sleep.