I have yet to see any proof that there were two off-duty police officers facing the construction site entrance on the night Brian disappeared. If off-duty cops were present in the bar, they were probably spending their time doing what off duty cops in bars usually do, i.e. hitting on the female patrons.
I also haven't seen proof that the doors were chained that night; someone could have forgotten to chain them altogether, or they could have inadvertently been chained more loosely than usual. The cops should not have assumed that they were in the same state on the night Brian disappeared as they were when checked several days later.
The two "people" at the top of the stairs by the escalator are Columbus police officers. They are hired as security and too keep the patrons to not go into areas that are closed and or off limits.