So one idea is Snapchat... once you post something it's gone a day later or once you delete it and private ones you can delete also and you can erase browser caches and someone doesn't have to be on your friends to send you snaps or view them assuming neither had eachother added or even she or he let anyone see their snaps and maybe some inside code word in his or her stories that's seems normal but everyone but even just heading out jogging ones so the other knows to head out and nobody is any suspicious because it's one they came up with in person at a bar or something. Maybe we should look into homes close with married or single men who are rich or attractive who wouldn't have motivw to kill if a secret girl threatened to tell. Or how many young dudes run routes but like it. And they could have hooked up but once or ages ago so her friends don't remember. Like my bestfriends forget some of mine. Or if her bf lives in town what if her and his friend had something going on. Those aren't the only ones but I don't think it was done by someone she was sleeping with. But it would explain why the sexual a assault thing that they stopped mentioning and depending on what they're into light chokibg could make them at first mention strangulation. But maybe now they're unsure and but hence we don't hear it. I mean she could have done those things with her secret bf or bf making them not realize it wasn't by actually rape or strangulation but maybe this person saw them meet up his anger grew me the suv of the dude leaves and she prepared to finish her run when some crazy obsessed dude attacks. Or this could he her an friend who's obsessed with her and took a chance when but the bf isn't in town(not sure where he lives) there's too many options. BUt to me I see jealous and obsessed stalker and if her bf was new then maybe that set off the stalker. ALL just ideas but s far as I know back to my first part and that is that Snapchat you could likely have a secret one via that. Maybe knowing how long she'd been with her bf could help some, could show us it set a stalker off.Your scenario explains why people get murdered every day. Rage, deception, one wants in and the other wants out etc. And you are right, people can hang out and not be seen, and it's possible to not tell anyone about it. Maybe he didn't buy her gifts.
How did they communicate?
How would they know when to meet?
I doubt they have anyone in mind, but if they do, why the DNA dragnet? Even if they have stopped in the past month (not sure if they have or not) They would have tracked that guy down through phone/ email ISP records within a week, and if he drove or had access to an SUV, they'd know that by now too, and I'd guess a DNA sample even if he refused.
Your scenario sounds perfect, except for three things. The first is the timeline and how they could have driven elsewhere first to talk, second, if they did why he brought her back there, and the third is, the electronic footprint.. I'm still not past that.