It was a subject of much mirth among the guests when Sherlock was forced to read out the fluffy telegrams at John and Mary's wedding in last Sunday's episode, The Sign of Three. But on closer examination it seems something far darker was lurking among the good wishes from absent friends.
Fans of the original Sherlock Holmes short stories will immediately recognise the acronym CAM as the signature of master blackmailer Charles Augustus Milverton, so when a message from "CAM" appears during the nuptials it immediately sets alarm bells ringing.
"To Mary, lots of love poppet, oodles of love and heaps of good wishes, from Cam. Wish your family could have seen this..."
The new Mrs Watson is visibly shaken by the message, prompting John to ask her if she's alright, while he seems completely unfamiliar with the mystery name.
It's clear that this is a veiled communication from new uber-villain Charles Augustus Magnussen, Mark Gatiss and Steven Moffat's modern-day take on the original CAM.
Lars Mikkelsen's character was glimpsed briefly at the end of the first episode, watching as Sherlock dragged John from inside a bonfire, and is set to reveal himself properly in the series finale His Last Vow.
But he obviously wants Mary to know he's around before then. Is Magnussen blackmailing her? The word poppet – puppet? – could suggest she is under his control, and the reference to her family, who we haven't met, is ominous at the very least.
"Sherlock Holmes has made one enormous mistake which will destroy the lives of everyone he loves and everything he holds dear," Magnussen tells us in the trailer for His Last Vow. Doesn't sound good, does it...?