Here's a thought:
* I believe Brian is alive by the way. This might seem far fetched but something similar happened in the UK*
What if Brian was tired, really drunk and entered the construction zone. Upon entering, he finds a box or even a bin to go to sleep inside of. It could even have been an idea for a prank (burst out of a box/bin jump scare) if he thought those two girls were going to follow him (they can't remember what they discussed with Brian that night) into there not long after he left the CCTV picture.
Brian gets into a box or bin to sleep or jump scare etc but something hits him on the head, for example a lead pipe or blunt construction object, that knocks him out cold or even kills him. When the construction work resumes, this box or bin is just carried out of the complex as normal before being disposed of in a land fill.
Sounds daft , I know, but something similar *apparently* happened to a missing airman over here in the UK. He *apparently* climbed into a bin after a night out to sleep in and was crushed inside a bin lorry, before being taken out to a landfill.
Can someone remind me, how long after Brian went missing did construction workers have access to the complex??
I know Brian's phone turned itself off like a minute or so after he disappeared from camera, but maybe he did that intentionally to go along with jumping out/scaring the girls? He would surely have known his phone would've rang as Clint and Meredith would've called, which would have given away his position??
Here is the missing airman and I say *apparently* above because his Mother insists the bin theory is wrong:
Corrie McKeague died after getting into waste bin, UK inquest hears
And here is an example of what I mean about maybe him (Brian) climbing into a bin or box to scare the girls. Please excuse the stupid mask, it's just the only one I could find.
Skip to about 1 minute 2 seconds:
I know this theory sounds totally ridiculous, but you just never know. Was Brian known to be a prankster?? Didn't one of the girls say she went into the construction zone when Brian was talking to them, to take a phone call?
Perhaps he thought she'd do it again and he could jump out at her as a prank. Or maybe he told them to come and meet him back there (if they don't recall what they were talking about with Brian then it's a possibility) after the bar had fully shut and they could leave together etc.
I get it's a daft theory but it popped into my head the other day, I obsess over this case!!