How do Hetty and Reese know that Byatt had been to the Harker's house in the novel, Wilder Girls?

Hetty and Reese see the initials “BW” carved into the birch tree at the Harker house. The carved initials are a symbol that Byatt was there. Byatt had a tendency to carve her initials everywhere she went, and Hetty sees these initials as hope that Byatt is alive.