I would contact the TCO fly shop in State College If you haven’t already done that and ask if anybody reported finding your backpack. It’s probably a long drive back there for you, but you might also consider putting up some posters near where you were parked and/or lost it.
As an aside, and I’m guilty of not always doing this myself, but it’s a good idea to mark all of our fishing gear, including fly rods, reels, vests, fly boxes, etc. with our name and telephone number in case these items get lost, which many invariably do from time to time.
Many years ago I lost a very expensive bamboo rod and reel by leaving it on top of the car one night at the tunnel parking area at Poe Paddy. I drove all the way back to State College before I remembered it, and by then it was too late. 30 years ago I lost a Sage SP rod and Abel reel by doing the same thing on the West Branch of the Delaware. You would think I’d have been smart enough by then to follow my own advice. As they say in these parts, some of us get too soon old and too late smart.