How unlikely is it that some agency already has people in place for these types of jobs, only to have that person plant an inconspicuous backdoor that can be re

How unlikely is it that some agency already has people in place for these types of jobs, only to have that person plant an inconspicuous backdoor that can be resurfaced several years later. I mean, if they're giving him access to the entire IT infrastructure, I'm sure it can be done in a way where it can't be pinned back.