When I retired, all my access to various local, state and federal criminal intel systems was eliminated. When I was asked to return as a contractor, it was restored. Whenever a federal employee leaves, turn his/her access off completely. That goes for POTUS, VPOTUS, Secretaries, Senators ... everyone.   If their expertise is needed and they are requested to come back, have them go through another complete security check and only then grant them access.