Billy doesn't necessarily spend all that much time around the community hall - but he just happens to be there at the same time as Scratch is working on his note, while Billy just came out of the storage with some thread for his projects.

He figures he should check the notice board on his way out, just in case there's anything important or useful on there, which means that he reads Scratch's note at the same time as the actual guy is still putting it up.

.. small talk isn't necessarily Billy's speciality, but he is kind of curious upon reading the note. Curious enough to look over at Scratch and ask: "Are you working on some sort of project?"