I too,am inclined to believe that she was missing before the 28th. The circumstances are too neat and compact for things to have happened quickly. It all seems too well planned that even the FBI, police etc. are stumped (unless they are waiting for more evidence.) I might be wrong, but it seems if everything happened in a span of a few hours, chances of more mistakes would have been evident by now.
While I am inclined to agree, I have to recognize that this would necessitate cover-up by MJ, DB, and KB. I believe that if all three were involved, someone would have cracked by now. The more people involved, the more likely one will have a conscience and speak up.
Something happening before the 28th would need....
1. DB to lie about seeing PB asleep that morning. Granted, she could have seen pillows under a blanket, and assumed it was PB--we all see what we want to see, and if she had no reason to suspect is WASN'T PB, she wouldn't have looked further. I am not sure (does anyone know) if DB actually made verbal communication, and got a sleepy mumble back? Or was it JUST visual?
2. DB and KB to lie about speaking WITH PB via phone, the morning of the 28th. Granted, someone could have pretended to be PB, and if both DB and KB expected it to be PB, they would have heard what they wanted.
3. DB to lie about the Facebook incident, and having PB sleuth. It would have been very strange, though possible, for DB to have that exchange with someone using PB's phone and pretending to be PB.
4. KB to lie about making plans with PB.
MJ to lie about everything that happened on the 28th.
Something happening before the 28th makes things more complicated. What's the phrase, about often times the simplest explanation is the right one? In this situation, I believe the simplest explanations are that either someone took her from the apartment, or she really did walk out (say she used the back door/patio, and thus didn't leave a trail...) and was abducted. All my opinion entirely.