Sycamore tells you there's a treasure for you somewhere in the town, talked to every npc, found all hidden items, still nothing (didn't go up the waterfall though, I don't have waterfall yet)?
>At the right lower bench, and the leftmost seat. Press A. You don't need the dowsing machine and it's right near the Pokeball.
It's past Sycamore writing a letter to future Sycamore on what kind of person he became, and asking him if he accomplished his dream and asking if he's living a good life.
I assume thats the treasure? At least that's what these guys say