You should have examined a sheet of paper on the noticeboard in the morgue's corridor. Sally and John were two lovers recently deceased.
Then in the shed that you unlocked with the keys, there's a document pinned to the wall. Be sure to read it in inventory.
Did you also examine the box in the middle of the shed and find the note?
You should have the map.
Those are the triggers to make Holmes comment on the Lovers' Tree.