In "The Final Problem," we learn of the unsolved disappearance of Redbeard,
initially presented as Sherlock's dog but later revealed to have been a friend of his.
We are told that the young Eurus had both arranged the disappearance and presented her family with a clue that would have revealed his location, and that when they could not solve the clue, she allowed him to die.
She gives the same (?) clue to Sherlock years later,
when she has hidden Dr. Watson in the same location.
This time he succeeds, finding the answer
"in my (Eurus's) room".
In what way could this possibly have been the correct answer at the time of Redbeard's original disappearance?