Backpack item?
Just throwing this out there as I haven't seen it discussed. I don't play lacrosse, but it does look like the item in his backpack could be a lacrosse helmet. Just google Cascade CPV and you may see what I mean. It also kind of looks like a bike helmet, but so do lacrosse helmets. It looks like it doesn't fall all the way to the bottom of the backpack, but it actually is because the lacrosse helmets have a black facemask that would account for that.
If that is the case, he did have his helmet, and if the item in his pocket was a water bottle then that would jive with him at least leaving to do something with lacrosse.
Speculating further....there has been no mention of this. There has been sketchy details about whether there was an actual lacrosse meeting (no statement from school, team, teammates that I am aware of). Might this be a hazing deal. He leaves to go to "meeting". Is cagey because he isn't supposed to tell. Phone is off because he is told to turn it off (no proof). Something goes awry.
If this was the case, I find it hard to believe that the LE would not have shook anyone involved. I can't imagine this would not have been determined already.
That said, the slow release of details, is either because their is none or because they don't want the public to know every little thing.
Wild theory. Could be completely unfounded, but stems from it looking like he had a lacrosse helmet in his backpack. Fits with all other info about him. Nothing odd about him taking part in some innocent college fun and something going wrong.