This is a really good example of info that LE for sure has, but isn't sharing. And there's so much more like that. They know the last time she used an electronic device, and the last time her Fitbit communicated with her phone. If she had her location services enabled on her phone (I think most young people do, as its convenient and they tend to be more trusting of that type of thing), then they know much more. If she had her Fitbit and her phone on her person when she was attacked, and the Fitbit was auto-syncing, then changes in her heart rate during the attack will have been captured. I think this is why we heard that they have a good timeline. A good timeline would mean, in my opinion, that they are pretty sure of when Mollie was taken.
And they aren't sharing any of that info. Not even to say "We have this data but can't share it due to the on-going investigation." Obviously just my opinion but I think its because they have a suspect, not because they are clueless.