Unfortunately news articles rarely have as much detail as we would like. It appears she didn't have it set to track but was just using to periodically check in. I'm semi-familiar with that area as it's not to far from the home of a friend of mine. If I had to guess based on the known facts I suspect she slipped and took a high spill off a rock face, or possibly drowned in a high-running river. The unit she has is pretty robust. Even if it broke or the batteries died she would have likely turned up somewhere or found another means to signal. I suspect she's incapable of reply due to injury/death from some kind of injury. Hopefully I'm wrong though!
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman