@DD8 I don’t feel confident that it’s the explanation, but it feels very plausible to me. She thinks she’s reached her destination, she’s safe, no need to be bothered by her phone.
And the timeframe is very short—less than a minute.
(I know that lots of people say that no one ever ever turns off their phone, but I don’t believe it.)
An afterthought. Do we have any reason to believe that AA knows people can be tracked by their phones? He certainly doesn’t turn his own phone off when he goes home from the park. He leaves his phone on when he takes her body to bury in Logan Canyon. He does burn her phone in his back yard—but that might just have been a by-product of burning her purse.