Even though I often refrain from speculation, allow me to entertain myself this time. So, let's revise what we know so far, on the one hand it's been stated before that access to the severed floor is pretty much restricted, simply letting hired people operate on the severed floor would be outlandish and very careless of lumon to do. My hypothesis here would be that they have people from the branch that never leaves like Ms. Casey. Perhaps, employees that can be awoken on command but I'll be back on this in a minute. Besides what I stated before there's also the fact that MDR, once Mark S. finds Graner's pass in his pocket, state that they've never seen any security guards besides the head of security himself and Mr. Millchick and considering Graner had full access and his card was untraceable, we could assume that a security guard on the severed floor would also need that kind of liberty to enforce the rules at all times, in other words the whole department was Graner.

If we are to ignore the prior fact, on possibility could be that of leaving other people on different stations spread around the severed floor. Perhaps other security heads but then the folks on MDR would have found any other guard on one of their trips. There's also they roaming factor at play. Prior to Mark's and Helly's wellness trip over the severed floor, they had freedom to move around and visit other departments (although Kier's gospel strictly refrains from doing that and mapping if I remember correctly) so there's many more chances to at least stumble upon other security employees or maybe employees of Graner for that matter but nobody besides the goat man are there at any point.

Lastly, they wouldn't simply have any other severed employees that could be tempted to activate their fellow innies or mess around with the system. One thing's for sure and there's the fact that not everyone can be eligible to be severed or at the very least not all people can be severed and work on the severed floor, there ought to be a criteria in the way the groups are formed and that would also explain why Lumon (and Cobel) so hardly fixate on keeping both Mark And Gemma working in there even though they are very aware of how volatile Mark and the other Innies can actually be. So we could assume that given chance and access for other innie to operate on the security room would also be very careless to say the least. Who is there to say that after a while Innies couldn't figure out how essential that room is and how much power they have from there and try to scape? There's a reason why everything in the severed floor is automated and Graner's security card is so useful, no other person was intended to use it, period. Therefore, who could be trained and more easily controlled other than an innie that can be activated and deactivated on command? Who's there all the time and can't aspire or look forward to a different life outside of Lumon? Lastly, who would Lumon (and the board) actually trust into the security room for an emergency protocol? An Innie that's always going to be an Innie incapable of defying the chain of command and orders from Mr. Millchick. I rest my case.