A coworker and I were once stuck in an office building for an hour or two. We were working as consultants at a client's building and ended up working rather late. Not particularly late by software programmer standards, but clearly exceptionally late by the culture of the client company.
At some point in the evening all the exit doors, including the front door, became armed, and this was conspicuously noted as when we packed up for the night and tried to exit to the parking lot, we realized we couldn't open the door without an alert being sent to the police (not just the security company). There should have been a guard at his station (desk, CCTVs, etc) in the entryway, but we found none.
We waited for awhile. Then we walked up, down, and through every corridor and restroom of that 4-5 story building, multiple times, looking for the guard. When that failed, we called the security company to ask them if it was okay to open the door. They swore there was a guard on duty and asked us to wait a little longer in case he was doing rounds. Despite knowing that couldn't possibly be the case, we obligingly passed more time waiting in the entryway. Then we walked up, down, and around the building again, but this time splitting up and shouting. Nothing. Nobody.
We go back down and inform the security company that we weren't going to wait any longer and that we'd be triggering the silent alarm as we left. And guess who exits the elevator just as we were about to open the door.... Apparently he had been sound asleep in a cozy nook somewhere in the upper floors--presumably in a conference room or more likely a private office, the former being something we inspected in passing (glass walls), the latter we didn't feel comfortable opening and entering, and both being the last place you'd expect to find a security guard. IIRC, he wouldn't admit it outright, but just played coy. We weren't mad. A little tired and frustrated because as consultants we still had to get in early the next morning, but that was mostly offset by the sheer absurdity of the situation, and by the fact that he seemed quite elderly.
Anyhow, you may assume too much if you assume the security guard actually maintains some kind of useful presence. I guess these days it's more common to have electronic way stations to log a guard doing rounds. I dunno if this building had such measures (this was circa 2001-2002), but as the sole guard he probably was expected to spend most of his time, if not all of his time, manning the security desk, providing ample opportunity to be doing something else, instead.