One time Sherlock came home with a matchbox he said they found in the apartment of a French decathlete. Found him half-crazy, surrounded by a thousand or so other matchboxes, all empty except the one Sherlock brought back with him. When you opened it, there was just—

[ John flounders here, looking for a word... can't find one. inexplicable, Sherlock had called it, and he hadn't been wrong. ]

Light, I suppose. Anyway, they took it off our hands within a day. Whisked it away to some secret facility, no doubt. That or it's sitting in Mycroft's junk room. [ ah. ] —Was.