Well, from what I gather, Annie's cell phone, laptop and I think an iPad are missing. But the cops will be able to track the towers that the cell phone texts went through. And if any of those devices have a "locator" option active, then they might also be able to track their last known location.
I also imagine they've processed Annie's vehicles and if either one was used to carry her body, they likely have that evidence, as well.
MM may be able to shield herself by pleading the 5th for now, but you can bet the cops have a mountain of evidence pointing toward her that she won't be able to deny for much longer. And the cops certainly aren't going to tip their hand by talking to the media about what information they have, lest MM destroy more evidence.
What's fuzzy to me is, why didn't she file a missing person's report or at least call Annie's friends or family to begin with? Did any of Annie's friends stop over at Annie's house when they noticed that she hadn't been around? Where was her best friend?
At this point, I'm thinking that the cops will continue to gather evidence, and unless they stumble upon something damning, they'll simply just wait for MM to screw up and/or confess.