That building had security guards. The article says that 8 of them had a chance to notice the security monitors, but none of them did.

The guy smoked some cigarettes, so presumably there was no smoke alarm. There was a manual alarm but no-one heard it and eventually he turned it off. There were engineers working in other elevators but they didn't check his. Although it was late Friday night, he had some work colleagues who expected him back, but they thought he'd bunked off deliberately.

He didn't even have a watch, so he didn't know what time it was or when other people were more likely to be about.