Hah, that's a pretty good jump between the last panels there. The simple explanation is that the guard just walked in from off-screen. The amusing explanation is that the Irregulars are doing a bit like Qui-Gon's sword and found a trooper to cut and paste into the scene! Seriously! Pete should have been quicker on the uptake though and immediately rolled some dice like Jim did, so I wouldn't be surprised if the trooper just vanishes again in the next comic while the GM and Pete argue about scene-setting expectations for a bit. An easy out for the GM though? Allow the roll to go through, then say it doesn't work because the guard has a cyborg brain or something. No fleshy brain, no Force Suggestion!

And of course, no guards doesn't necessarily mean not secure. Sensors to check for odd movements, sensors to check for footstep pressure, sensors to check for IR signature changes... Set any of those up to the atmospheric controls for the room and vent sleeping gas in if any of the sensors trigger. Remove all the door controls from the inside of the room and, while the First Order Fire Marshall might have a fit, you've got a room that would be extremely difficult to escape. Even Jedi would need to breathe at some point!