In tonight’s episode, there was an exchange between Sansa, Littlefinger, Yohn Royce, and the new Maester of Winterfell Wolkan. In this exchange Sansa asks about how long the longest winter in the last 100 years was. Wolkan claims he doesn’t know, that he will check the records left by old Maester Luwin. He goes on to say that Luwin kept excellent records of every message he received. This causes Littlefinger to give him an intrigued look. Why would Littlefinger be concerned about the records of letters from Luwin? Because of one letter. The letter Lysa Arryn sent her sister Catelyn in the pilot.
In this letter which is delivered by Luwin, Lysa makes the claim that the Lannisters killed her husband Jon Arryns. Catelyn immediately burns the letter but goes on to say what it said in front of the Maester and Ned. From this conversation the War of the Five Kings had its roots as Ned and Catelyn go looking for proof of the murder. Why does this matter? Because Lysa confessed in front of Sansa during the Moon Door scene that she did it.
It’s unlikely that Maester Luwin made a copy of the letter, although it is possible to break and replace a wax seal without notice. He kept excellent records though. The show is hinting that there is a record of that letter, from Lysa Arryn to Catelyn Stark right before the War of the Five Kings. A woman Sansa knows was obsessed with Littlefinger, that almost killed Sansa over being kissed by him, who said that her husband got between her and Petyr.
This may be the beginning of the end for Petyr Baelish. Why? The Streissand effect.
The Streisand effect is the phenomenon whereby an attempt to hide, remove, or censor a piece of information has the unintended consequence of publicizing the information more widely, usually facilitated by the Internet. It is an example of psychological reactance, wherein once people are aware something is being kept from them, their motivation to access and spread the information is increased.
Petyr noticed that there are excellent records kept of all letters to Winterfell. He knows if Sansa discovers he is behind the plot that let to the death of her parents, her brothers, her sister (as far as she knows), her friends, her countrymen he will be lucky to have a swift death. So he must destroy the evidence somehow, get those records. And in doing so he’s going to be calling attention to their existence. Luwin may have erased the record after the conversation he had with Cat and Ned never made them at all. As they say in that room, the contents of that letter is incredibly dangerous and Cat burns it. But Littlefinger doesn’t know that and if a record of that letter might exist, he has to get rid of it. Causing him to draw attention to why he even cares about the letters Sansa’s parents received. And making Sansa put together the pieces in a way she never would have had he done nothing. If he makes any move on the records, Sansa will remember the only person with her when Wolkan told her about the records.
TL:DR Littlefinger may try to prevent Sansa from learning that Lysa Arryn sent a letter to her sister Cat about the murder of her husband. In doing so, Littlefinger is going to call attention to these records being important to him either by being caught trying to get them or having them destroyed. Which may finally lead to Sansa putting it together exactly who was behind the downfall of her house.