In S01E01 A Study in Pink, Sherlock discovers that the killer kept the victim's phone and decides to send a carefully crafted text message to it. He sends, "What happened at Lauriston Gardens? I must have blacked out". Along with an address and "please come".
Sherlock explains that the killer, after receiving such a text, must know that it could only have come from his victim.
Isn't this a really strange thing to do as a victim that woke up in a strange place without their phone and (apparently) their memory? Most people would assume the phone was lost and if anyone had it at all, they'd be a complete stranger. Why would you text such a thing to a stranger that found your phone?
One explanation could be that the victim thinks a friend must have grabbed their phone. But the victim in this episode was on a business trip, presumably traveling alone.
At this point in the story, Sherlock doesn't know the killer is intelligent. Does Sherlock purposely send a text that doesn't really make sense assuming the killer will panic and not think it through?