Here is something that I can not fathom.
Her dad called pretty soon, maybe she was alive. No answer, and with all subsequent calls, the phone would go to VM.
Was she so smart as to switch it off? Or put on “mute” and then it lost power? Lots of photos, a video, lots of battery life lost. Maybe he did see it but with the power being off, decided it was off for a while? Because taking the phone with you is a huge burden, it starts traveling with yours.
Perhaps he wanted to avoid the liability of carrying her phone, and since it was dead, merely left it with her?