"I imagine she is very good at it," Sherlock puts in, dry as dust.

"You know, I've never met Mrs Chandler, but if this shining example is anything to go by, I think you're right."

Now, on the surface of it, this is a fairly harmless compliment. He just called Cal charming. But for once, there is something Sherlock doesn't notice, and that is the unfortunate implications of the way Obadiah said it.