This is not really surprising, as he seems to be at every game, but here's some video proof in case you needed it.
In case you're wondering, yes, Jed York was up in Seattle for the San Francisco 49ers 29-13 loss to the Seattle Seahawks. Some random Seahawks fan was up in a luxury suite, and noticed that Jed was in the suite next to his. He shot this quick video of Jed, sitting next to his dad. Jed left with about four minutes left in the game, presumably to go down to the locker room.