If she only did that once in many hundred days, on that night in particular, then that could be a very huge amount of evidence.
....No. Not even slightly. This line of questioning MIGHT be relevant if you didn’t already have the killer identified, with overwhelming physical evidence pointing towards them. You don’t need to explain why Knox turned her phone off, because you already have the killer and every single piece of physical evidence at the crime scene accounted for.
Slate Star Codex fulfills this niche for me.