As a former 2nd grade teacher, I really feel for Kyron's teacher! It's every teacher's nightmare that something will happen to one of your students when you're supposed to be in charge. A nightmare!
I can't help but think that she was less concerned at Kyron's absence than she might have been because the last time she saw him, he was with a parent. She definitely noticed he wasn't there, and marked him absent, but the idea that he could be snatched by a stranger in between being with a parent and the start of class was so out there.
Probably she was overwhelmed by the logistics of the science fair, too. And attendance matters are usually overseen by the front office, so she just assumed he went home with the SM, marked him absent, and moved on. When I was a teacher, I never ever called a parent during the school day -- so even if I was very concerned, I would have waited until after school to call.
I hope this poor woman doesn't feel too horrible.