I am not in security, but I have worked in secure areas. The way you prevent issues is having multiple layers of security that watch each other.
Like you prevent individual employees from committing fraud by having other employees sign off on their work. Then you prevent those employees from colluding to commit fraud by having another group of employees monitor their actions. Finally a third group of employees audits everyone occasionally (at random).
This way it requires at least 4 people who don’t know each other to do anything illegal. I’m sure the Secret Service could do with some audits. Like literally have an entire team of Secret Service people test them, trying to trick them into making a mistake.