If they do figure it out, they will have to take down the entire system and spend days reinstalling software to remove your hacks. They hate this. You horrible little man. If facing such a defense, the hacker will need to breach them in order, hacking the outer network to gain access to the inner ones. As a simple abstraction, assume hacking an outer network to level 4 will allow attempts to made on the inner networks.
Few networks are more than three levels deep.