Factual error: There's more PIR's (movement detectors used in security systems) in Hireclere than you could find in a security warehouse! Just look behind the actors in the corners of a room about 8 feet up. White plastic things about the size of a packet of smokes. I think the earliest units on the market started appearing around the early 70's. The PIR's in Downton Abby look to be around the late 80's onward - pretty advanced for 1914.
Continuity mistake: Every letter's handwriting looks the same - no matter who wrote it.
Character mistake: At one point they say the gramophone was gift for Sybil's wedding. Later they say don't play the gramophone from Mary's wedding, it will make Edith sad.
Answer: Word gets around. The staff are gossips amongst themselves, but those loyal to the family know who to keep secrets from and who they can trust to share in the knowledge.
Phixius ★