Yes, I'm aware of what "possibly" means but I'm not debating that. I also know what "likely" means and to me it stretches credulity to suggest a theory, while theoretically within the realms of possibility, is "likely" that has absolutely no evidence to support it. Is it technically possible he could still be in the building? Yes, but it seems to be an incredibly slim one and is a theory that only lives on due to the lack of evidence, not because there's any real evidence to still put him there. In fact, what evidence we do have, though scant and not conclusive, works against the theory.
In addition, even though Hurst now speculates that Brian exited via the construction area - he was never seen on the camera heading in that direction. The same camera that did capture the band leaving by that back hall. So the lack of being seen on camera that is used to suggest he never left also works against I'm being in the area some people think he still hidden in.
Out of ALL the people that were through that area, just in the first week, you don't think that someone would have mentioned if cement was freshly poured or would have investigated that fact if it had? Never mind the police, numerous other parties were also through there ...including Randy Shaffer... you don't think his father would have made a fuss if concrete were poured in the middle of searching for Brian?
Also, I posted a link to news footage a while back that offers a brief glimpse of the construction area in question. It's the best view we have of that area so far, though it's not much. However, if news crews were able to survey the scene before floors were finished then that also goes against the idea that cement was quickly poured.
While we don't have searches logged down to the exact date and hour, there's more information available than you suggest. Just going off the original NBC4i reports from the time he went missing, you can piece together how exhaustive the search was there, both by people and dogs, within the first 7-10 days.
Not just in the gateway building that housed UTS but also the surrounding area and the local trash dumps were searched with dogs.
"concealed"? What if they just haven't belabored the point because none of them believe he's still there? Hurst said they have three theories of what happened to Brian and won't talk about them but he has no problem discussing Brian maybe going through the construction area and possibly exiting there.
It's interesting, and can be potentially helpful, to speculate on how things might have happened but when you base a theory on unsubstantiated conjecture (such as "maybe they know that cement was poured on Sunday or on Monday morning" or "cadaver dogs weren't taken to the site until a month later") then you should at least try to back up the conjecture before you use it to state a particular theory is "likely".
I'm not suggesting this theory shouldn't be entertained, explored, or discussed but when I ask if you have any info to suggest when the concrete was poured and you resort to suggesting it's being "concealed", etc, then it doesn't seem very sound in the face of what we do know.