UPDATE: PACK RETURNED!
Posting for a friend: 8:30pm Saturday night his backpack was left at the pullout across from Cutthroat at WA Pass. Returned at 10:30pm after realizing it was left behind, but someone had picked it up already. It was one of the three guys in the yellow van. Please, hit me up if you know its whereabouts! Notes left on cars and at the Mazama store. Bummer end to their fun day!
https://www.mountainproject.com/forum/topic/118111639/washington-pass-arcteryx-backpack-with-ice-climbing-gear