Perhaps it started at that specific - til now not to be seen - moment, when Haibara bought the last Higo charm. The culprit could be behind Haibara and her companion(s) at the store. Although I don't know, how the culprit could follow them during the match, but it could be seen at the train on the third page right panel. The orientation of his stand was sideways in order to look occasionally to Haibara, looking perhaps the moment for catching the Higo charm. But it was this sudden stop, and the third persona in this case picked up the charm.
He fabricated a plan, in order to get that specific charm: he left a very look-alike charm1 at the lost-and-found2, because he knew, that Haibara (or perhaps DB-s) would realize, that it's not the charm, what they looking for. And they would go on from this specific place, which is the only reasonable place3 to leave the lost and found objects. From this point it's easy, he has only to follow them secretly again.
And we could see, that he stole this charm at the recent spoiler pics...
1.) He bought the look-alike charm perhaps at the store, and his purpose was originally stealing at the way to home, and dropping the "fake-one" on the runaway to gain some time.
2.) The man at the lost-and-found said, that there was a much younger person (, than the person DB & Amuro looking for), who left this charm.
3.) But I wonder: why did the culprit leave at Chiyaricho station?
Finally, my thoughts about this: I cannot decide whether it's an odd (and a unique) or silly case. It contains a complication, but I just don't know. I am looking for the whole file...