Glyn Burgess suggests the branch is merely a signal Tristan has already told Iseult about in an earlier message; the poet indicates that Iseult would be on the lookout for the branch, "for this had all happened before".
The reference indicates that Glyn Burgess suggests the department might be a topic Tristan had mentioned to Iseult earlier, which she is already aware of because this type of situation has happened before.