This is frustrating for me.
I read this sentence:
On Central Avenue they came to a small inexpensive hotel, the Atlantic House. As they had done at all the other hotels, they asked the clerk if they could see his registration book.They turned first to Friday, September 28, 1894, the day that Holmes, while already in possession of Alice, had picked up Nellie and Howard from their St. Louis home.
The detectives are trying to find this murderer who allegedly killed three children after taking them away from their mother, and the detectives are tracing him.
This past perfect use shows us that Holmes's picking up the children should have had some important relation to the past reference point.
It is possible to theorize that Holmes's action had relation to the detectives' turning first to Friday (by making the day that detectives turned to Friday the past reference point), but it seems too distant.
I think that the past reference point is the point at which Holmes signed in after picking the children up on the same day, but in the context, it is not yet revealed that Holmes actually checked in, even though by the nuance of it we can pretty much figure it out. Also, the past reference point is never mentioned directly,but only implied slightly.
Is it valid use of "had"? Is it fine to use past perfect with so slight an implication for the past reference point?