With cell phones and clouds these days, it's not very easy to hide your location from Google or Apple, which means the government is going to know where everyone is if their phone is on. And if someone sent a text from the phone, they're going to know where the phone was.
That doesn't mean they'll be able to find the girl, but just following his phone, and her phone, they will determine when and where they started diverging. That's where they'll look first. And that's probably how they'll trip him up. If he lies about where he was at a particular time, they will definitely turn up the heat.
I could get Laundrie to talk, but I would need Hunter Biden type immunity first.