I'm starting to come to the thinking that CB was already in the apartment during one of either GM or MOs checks. Bear with me on this.
HCW was asked in the recent interview about whether they had any evidence placing CB in 5A. His comment was that he could not say, since either answer would invite a thousand more questions. At the time, this seemed a reasonable thing to say, but thinking back on it, would saying "no" really invited many questions other than the obvious "what do you have then"?
I think everyone assumed they had no such evidence anyway given the importance they are placing on finding the mystery caller. And if they had evidence of CB being in 5A, it would make placing him on that call a moot point you'd think. But then, it depends on what that evidence is. And like HCWs comments on the possible video, I'm begininning to think they maybe do have something.
It led me thinking to another comment HCW made, early on in the investigation. He said that CB had knowledge of the crime that only the suspect/abductor could know. For them to know CB had that knowledge, CB must have told someone a detail about the crime. Most likely it was something said in the confession to HB. But when you consider what this detail could be, the options are limited. For "only" the abductor to know, it needs to be some piece of information that is not in the public domain. It also needs to be something LE can actually confirm took place.
Given that there is no trace of MM beyond 5A, it would surely have to be something that happened within 5A. But again, there seems to be nothing relating to the actual abduction sequence that LE could verify, since nobody saw what took place. All the available evidence led the original investigation team to doubt that an abduction even happened. MM effectively vanished without a trace.
The only sound theory I can come up with is that CB was in the apartment during either GM or MOs check, and that CB has told HB a detail about what one of them did while he was in there. An act that was not mentioned in any of their statements, possibly because it was such a minor or irrelevant detail. It could be something as inane as them pouring a glass of water or whistling a tune to themselves while CB watched/listened from a hiding place. It is something though that LE could have potentially verified as being true via some discreet questioning of the involved witness.
It seems more plausible than other options discussed previously such as CB talking about a birthmark. I can easily imagine CB boasting about hiding right under their nose while they did x, y or z. I can't really imagine why he would go to the detail of mentioning a birthmark to HB. It also doesn't really fit with "knowledge of the crime" (he could have seen a birthmark in a video from someone else), plus other people would have known about a birthmark.
I could be wrong, but it makes sense of HCW comments, as well as many other aspects of what's gone on and how FF has gone about his defence. It places CB in the apartment but unfortunately, it's possibly not robust enough evidence for a trial. The defence will claim (and are claiming) that HB is a liar and could dismiss this knowledge of what happened in 5A as coming from another source. FF could also challenge the original witness testimony and ask why the detail was never mentioned in their previous statements. If however, LE could combine HBs testimony with a seperate witness placing CB at the scene via the phone call, that becomes extremely compelling when put in front of a judge. And it would make a lot of sense as to why they are placing so much importance on tying CB to that call.
All JMO, feel free to challenge.