Probably he just forgot and cycled the bolt (which becomes pretty much a reflex after awhile) and didn't even notice the brass flew out the truck window. Hopefully they'll be able to catch him from that. It's not just the firing pin imprint. . . they should also be able to trace the ammunition by the "turnings" on the sides of the brass, from manufacture through point of sale. That was the key evidence in the re-investigation of the Sacco & Vanzetti case, BTW.
Holding up a round, the reporter states, "it's a high caliber weapon"
That may be a second purpose for the box on the back of the truck; I haven't heard any reports of anyone hearing shots being fired. That box may be serving as a live-in suppressor/brass catcher. There is nothing to tie the discovered brass to these shootings. Could even have been fired from a different rifle and tossed as a red herring.