If they'd had time, and Sherlock had had the patience, he could have told Joan the stories of the phones and the people who had given them to him: dumpster divers, junkies, unsuspecting hotel managers who had left him alone with the lost-and-found box.

When he got right down to it, he used the phones to save lives - mostly his, nowadays, he could think of at least three occasions where he had avoided a messy death by tossing a phone and missing a scheduled meetup.

There wasn't a life to save here - he felt it in his bones (and his mind, always the mind, he just hadn't bothered to uncode his suspicions yet); but he sent the message anyway and didn't bother waiting for an answer - Joan would get that soon enough, and he doesn't think she'll like the answer.