Usually, I'd agree with you about the phone. However, a mom in the situation I think SM is in would certainly behave very different to a regular mom. Indeed, there are still people (few) who don't have cell phones and there are people who aren't allowed their cell phone while they work (and their kids are in school).
At this point in a case, I like to ponder how much SM might know about cell phones. Quite a bit, I'm guessing. But anything she did (turning it off, putting it on airplane mode, leaving it somewhere) is going to be evidence. It's going to help persuade a jury if it comes to that.
Likewise with the GPS on the truck, if any. I'm sure LE is getting or has gotten records on both. Apparently, she was cooperating with LE at some point, but I'm guessing she has not given them any sort of statement about where she was for those 4 missing hours. Hopefully they've got a crew looking for further CCTV of the truck as it turns from the development and as it heads where ever it was going.