Thank you so much for the kind words ^^
Given that the upper floors add sentry guns to their security arsenal, then I could definitely imagine LunaTech having installed some on the roof, exactly for that reason. Or drones that scan incoming aircraft, in case of the PCs dropping via heli or similar. Something for the PCs to deal with before they actually get boots on the roof. Or maybe their drop unto the roof is a one-way street (like the heli pilot only being able to drop them off, but not stay, because of the drones and/or sentry guns from earlier). Something to give the players that signifies that they are going through a fortified backdoor, where they do not have the clearance to be. Now it can be a mission heavily leaning on actual don't-get-seen stealth, and because of that, the alarm ticking up is potentially an even more potent threat. Hope it helps :)
Edit: clarifications.