I'd love to see the vid of Clint and Brian going down the escalator following their first visit of the evening to the UTS. I'd bet that it was very easy to identify Brian both up and down on that first trip, just as it was very easy to identify him up on the second trip.
That said, I don't think CPD missed Brian - tall, slender, long-sleeved Tshirt under polo shirt - going down the escalator a second time, I think he simply did not come down a second time. If I understand correctly, the analysis showed that X number of people went up the escalator, and every one of them was accounted for going down - except one. In other words, it wasn't like say 432 people went up, and 432 came down, and one of the 432 who came down didn't match with one of the 432 who went up (that would have raised the question, who is the surplus person seen going up but not seen going down - CPD would presumably have shown us the image of that individual and asked 'who is this person?'). No, it was like 432 people went up, and only 431 came down. Where it gets interesting is that #432, the one seen going up but not seen going down, turns out to have been the individual who permanently disappeared that evening - Brian Shaffer. Coincidence? I think not. I think that Brian's seeming to not have left the building that eve/morn by conventional means is linked to his permanent disappearance and very likely death that night. Linked by causality. Whatever caused him to not leave the building caused his disappearance and death. Which would mean the entire case was just a stupid drunken accident minutes after Brian was last seen on vid, leaving the body hidden from view, with a failed search of the premises.