Just hit NG+1 and sped my way to the Undead Settlement. Once I arrived at the Giant Tower, I decided to get Seigward to activate his initial lift-descend animation and run back to save 'Touch Me' blind lady. In short, ran into tower, ran out as soon as the lift started moving. About 10 seconds later I get the 'whoosh' of souls sound and figured the giant got some kills or something fell down the nearby pit. Saved 'Touch Me' lady and proceeded to the tower, expecting to see Seigward. He wasn't there. Went down, got my frosty sword and grabbed the bonfire for next area, went back to the top, "I help any time", went back down to Old Demon, still no sign of Seigward. It was then that I figured that 'whoosh' of souls was him. Reloaded the area after clearing it, went back, and there it was - Seigwards Stormruler, Shield, and whole set, right by the lift.
I have no idea how that happened as he is scripted to be on the lift until it arrives, walk a few steps off it, and wait until you chat/leave, but alas, I assume running from the tower a sufficient distance probably despawned the background/lift and he randomly fell to his death?
Oh well, no Titan Slab for me in the Prison this run.