It's like you said, both left to get Austin from Solitary. One guard (who wanted the carbon from Gavin) left "to tell them we're ready" - he probably went to the bus area. You hear a lot of footsteps running around as the alarms go off, female guard and unnamed guard (We're all good over here") helps to lock up the "lockdown-gang", and then female guard says "central called for help in 8 block", so then you hear a muffled "come on lets go" from unnamed guard, more running footsteps, and then Mark yells LOCKDOWN after. So I don't think someone's left out.