Stardew Valley Expanded changes three of the secret notes from the vanilla game, and uses one to trigger a new vendor.
For more information on the original Secret Notes, pleases visit the main wiki.
Secret Note #18[]
Due to changes with the Calico Desert, SVE has changed the original Secret Note #18 to reflect the new map. This note still points to a spot in the desert where you can hoe the earth to find a Strange Doll (yellow).
Secret Note #19[]
In Stardew Valley, Secret Note #19 is a treasure map through town, starting with Jodi's house and using arrows to point the player to a hidden item. Since SVE edits the Town map, the treasure map had to be changed as well. The item and it's final location have not changed however.
Secret Note #20[]
In Stardew Valley, Secret Note #20 is another treasure map. Like #19, it was changed to fit the new Town map. Rather than make another treasure map, SVE converts it into a note calling you to the final location.
Secret Note #23[]
SVE doesn't change this note, which directs the player to bring maple syrup to the Secret Woods. But once you bring the maple syrup to the Secret Woods, a new shop appears in Forest West.