Damn, now I want to run a run in a late-night office environment. I did that for a week and wish it hadn't ended. Let's see... Some Characters they may find.
1: The Newbie. He's young, enthusiastic, it's his first night on the job, and he's never had access to coffee (or soy-kaf) this strong before or in this much quantity before, and he's been hitting it hard. He's completely wired and resistant to stun damage, and being bright-eyed and full of youthful naivete, may confront the runners if he suspects something off, or try to go Stealth on them and tail them or sneak/hide away to call for security/the cops with his commlink. He's also committed some important passcodes to his commlink's memory because he hasn't yet memorized them.
2: The Grandmother. This kindly old woman is in a slightly-elevated managerial position over the pack of bottom-rung drones and has the unenviable task of keeping a bunch of teenagers, twentysomethings, and twentysomethings who still behave like teenagers in line. She likes them all, but if she feels things are going too slow/not being taken seriously enough, she restores order by picking someone who's doing everything the way they're supposed to (if there's no obvious cut-ups, that is,) and laying into them, telling them that if they aren't going to take everything seriously and do their job, she'll send them home then and there. She might mistakenly pick the runners for this. How do you deal with a kindly old grandmother getting stern with you and telling you to get into the pool and do some work?
3: The Joker. He's entertaining the other bottom-rungers by weaving a freeform tale of complete drek while everybody but the bosses is around one table, working on their links. He's loud and has everyone's attention as he spins a tale about the 'mongols of the deserts of southern Florida' or whatever. Runners may need to make a composure test to not stop and listen to his captivating tale of complete nonsense; he might make a good distraction, but there's a chance that the Grandmother will spot them when she goes past them to tell him that story time is over.
(The Newbie was me, by the way. I was soooooooo wired on coffee that night I achieved a chemically-altered state of perception on the 60-mile drive home. Thank goodness the highway was empty at 4 in the morning. I will neither confirm nor deny the based-on-a-true-storyness of the others.)
Other than that, I like the suggestions that have come before, especially if you consider that this late-night graveyard shift is packed full of cut-ups and jokesters that the company can't quite come up with justification to fire and so consequently they get away with damn-near everything under the sun. Depending on how much oversight they have, it could be anything from a corporate powder-keg where a much-reviled boss is on the 'frag list' should anything actually occur - and thus the 'runners will actually be aided by a storming mob of angry office workers ready to take any excuse they can to shoot their boss and blame it on outsiders - to a virtually-uncontrolled late-night madhouse where the official policy is 'we see nothing and hear nothing as long as all the work assigned is done by morning shift's arrival.' So you might have employees racing hacked Ferret drones down the corridors (with or without office workers astride their overworked and much-abused steeds,) or the gun nut from security who spends his every free moment either watching action trid flics, on the range, or customizing his personal gun and is just
itching to go John McClane on someone's ass.
The security spider might be bored and hate his job and his first action upon seeing an intruder in the system will be to offer to sell them an admin account he made and caused to look as if it were hacked... Or he might be a sociopathic, homicidal
Bastard Operator from Hell who already has a significant body count to his name, consisting of employees and superiors (and executives, and insurance agents, and investigators,) who refused to see things (like the budget) his way and who won't tolerate outsiders breaking into his castle like this. Expect lift doors to open onto empty shafts and ridiculous booby-traps, or any lift they use will take them to his personal dungeon in the unused sub-basement and lock-down, forcing them to try to navigate a hellish booby-trapped nightmare, and of course he has an up-amped, highly customized version of the most powerful melee stun weapon in the books sitting in the drawer of his desk... And a claw hammer. He'll also try to lock them in the backup safe (which has a faraday cage installed,) or trigger the Halon-equivalent fire-suppression systems. Of course, he's always susceptible to bribery, in nuyen, electronics goods, sex, or booze, but his price will go up the more angry he is with them (and the more he has them in his clutches.) He might not care if they off the Helldesk assholes who are thorns in his side, and the beancounters who are always trying to get rid of him, but he has a good working relationship with security, based on mutual bribery and backscratching, and his current boss is an idiot with a bad memory and a fetish for farmyard animals who can be easily bamboozled into signing anything or believing he approved of something some time ago, or blackmailed if bamboozling doesn't work, and the BOfH will be extremely upset if they kill him and thus make him need to break in a new boss, which usually involves the discrete murder of three or four intermediate bosses who aren't quite so rubbish and easily-fooled and easily-blackmailed.
You could also throw in an Undercover Boss scenario; either the CEO of the subsidiary company (who is himself a mid-level executive with Ares Macrotech), or even Damian Knight himself, is undercover in the office pool under an assumed name. This might lead to trouble later-on if they go in with guns and act like the Pink Mohawk Gang (unless they successfully convince everyone it's another Code Green Live Fire Drill and that they're not in any genuine peril,) but if they pull the job with professionalism and cleverness, it might lead to further job opportunities. Or they might shoot Damian Knight right in the god-damn face and completely change the face of the game world.