I can only imagine how very heartbroken Angel's people are, in addition to the writers of 24 Paws of Love (who were Angel's breeder, if I understand correctly).
Angel's owners did a lot of things right as soon as she was missing. They got the word out, and they searched for her tirelessly. Equipment failures happen; if Angel had never before slipped her collar, there was no reason to anticipate that she would at that time. There are any number of things that startle a dog into running. It's a sad fact, and my heart hurts for them.
"Collar security" as such didn't initially occur to me as a beginner dog owner. Elka in fact slipped her collar once, but it seemed more like it was for fun than any other reason. I talked about an equipment epiphany when I got Elka's new collar, and also Liberty's Attachment, and an EzyDog Harness. I self deprecate myself a bit, calling myself paranoid and a crazy dog lady, but stories like this make me want to wrap Elka in cotton wool and never leave the house ever again. There's just no anticipating what can happen in a day.
My thoughts are with Angel's family, and I hope they heal well and soon.