A few thoughts after reading all of the threads:
The school looks like it is built into a hillside. So it could be that a 2nd floor room actually does open onto a back parking lot. Or it could be bad reporting (as we've seen a LOT so far).
The 2nd floor classroom could be why TH didn't walk all the way to the classroom. If she had Baby K in a stroller, she might have just said goodbye to him at the stairs instead of going back up among the crowd. Not to mention, despite what some commenters on Topix think, I wouldn't hesitate to let my 2nd grader walk to class alone. I have dropped my kids at the curb (as requested by the school) since they were in kindergarten. Also, does the "dood" think the bus driver walks those kids to class every day, or do they manage to get there on their own?
Baby K's nap schedule might be why TH didn't go to talent show. I know that when my own DD was about that age it was a nightmare if she was off her schedule. She was too old to sleep in her carseat and too young to miss a nap one day. There were times I reluctantly missed an event b/c I knew she would cry through the whole thing and I'd have to walk out anyway.
Baby K and her schedule may also be why TH is sometimes on fb when others apparently think she should be beating the bushes for Kyron. She may be "trapped" in the house taking care of her DD, feeling helpless and unable to do anything, and going online is her only outlet. If her DH and LE are at the command center, she may literally be home alone while the baby naps with nothing but the computer. I would be unable to resist the temptation of reading what people are saying, looking for updates, etc. And I might also get angry and frustrated enough that I would lash out and make a comment when I really wasn't supposed to.
At school, clearly there is the policy and what happened. I can't blame them as I have had the same experience of the sign in/out procedure being tossed out when 100 people are standing at the window. They just open the door, no badges or sign in. It has never concerned me because I think someone would have to know that to take advantage of it.
I am also very familiar with kids coming for just part of a special day at the end of the year when there are no other academics really going on. As a teacher, this wouldn't faze me. And while Kyron may have been in the talent show, I don't know that everyone would know or remember that. Like the person who does attendance, for example.
Also, I can totally see TH mentioning to the teacher (in the middle of the crazy gym!) that Kyron would be late on Monday or whatever b/c of an appointment, and the teacher not really paying attention (justifiably - trying to watch the other kids) and only remembering later "Did she say something about an appointment?" ~shrug~ She may have even said to TH, "email me so I don't forget" and TH did.
As for being called if the child leaves something...I have never. My kids have left lunchboxes, homework, PE bags (with their only tennis shoes!), etc. I would not expect a call, nor would I have called when I was teaching. It may not have been noticed until all of the other bags, etc. were taken home, and then just assumed that they forgot.
It is easy to armchair QB the situation, but seriously, like everyone keeps saying, when has a child been taken from a school? That cuts both ways - makes it seem unlikely, but also makes the school's lack of panic and TH's trust very reasonable.