I was thinking about situations like this :
Dipper can’t see Bill and think that he is dead. But when he is coming back to Gravity falls ( when he is 21 years old) he know that something is wrong. He couldn’t say why, but he feel it and he must to find out and save the world…again
Soooo, Dipper couldn’t see Bill,but is feel his touch.
His sister notice that something wrong.
-Are you ok, Dipper?
-Yes. No"…I am not sure, but I think I feel…strange? I don’t know.
-Well well well, look what I found - little Pine tree is growing up, isn’t it? Such a big boy you are now, Pine tree. And you can play in a games for adult, isn’t it? Let’s play, Pine tree. Oh, don’t you worry, it would be our secret, hmm?
Bill is talking to himself, Dipper can’t hear him…for now .