Factual error: A scene labeled as being from 1951 shows a name (Nook) having been typed on paper and then being whited out by a substance similar to Liquid Paper. Liquid Paper wasn't invented until 1956 in an American kitchen and wasn't marketed, and the patent applied for until 1958. (00:20:01 - 00:20:25)
Factual error: In the opening scenes the staff car door is opened by a four-ringer Wren (Deputy Director, very rare) equivalent to Royal Navy Captain, instead of a rating.
Chosen answer: This is not a mystery. Claire "stole them to read them" as Tom Jericho told Hester. As he later explained to Wigram, she had taken the messages to give to Puck who had the means to decipher them, and who was looking for his missing brother. The Kestrel traffic from ADU contained the names of victims of the Katyn Massacre. However, Puck and Claire were surprised by the imminent reappearance of Tom, and suddenly fled, explaining why some messages were left behind.