help me please with this --

Imagine that some group of the guards in a prison got mad and shot each other, therefore some
of the prisoners managed to escape. They are approaching the dead bodies of the guards. One of
the prisoners is saying:

"Some guards here,
but this can't be all of them."

"Check their SLOTS in the back...
and be careful."

What is SLOTS in this context?

Are they mechanical or cyber-guards?
No, ordinary people.
Students: We have free audio pronunciation exercises.
Are they wearing body armour?
I'm afraid not.
Could the slots be the places where the guards sleep?

(I think I have 17 questions left.)
Students: Are you brave enough to let our tutors analyse your pronunciation?
That would seem to make sense - checking for other guards not on duty.
And maybe these are the places where the guards are?
So, not the guards' backs, but 'back' as in not the area on view: in the back or round the back or out the back.
Teachers: We supply a list of EFL job vacancies