User blog comment:PurplePenguin/The Grunkle Stan Mystery/@comment-76.172.124.34-20120922184143/@comment-71.221.161.57-20121231021827

He did! Check out episode "The Legend of the Gobblewonker". When Mable is poking/teasing Dipper in the woods on Scuttlebutt island, you can see slender man in the background. Look carefully!