_{anon has only vague idea about Ghost story and haven't read preview on principle}
Since Harry is a ghost now no mortal boundaries can stop him. He takes some time out of his busy afterlife existence to visit Marcone - maybe to spook him or just to see what is he up to now that Harry is dead. He is surprised to find John if private mourning.

Further development is up to author but I'd prefer that in the end Marcone finds out about Ghost!Harry and helps him return to life.