Daniel thought Jeremy hadn’t eaten the cookies, since the food bag was still awkwardly bulgy when Daniel carried it after Jeremy.
The only one who didn’t have trouble with the bag was Toby, who carried it last, took it away alone into the woods for his ‘pit stop’, and then announced that it wasn’t heavy or awkward. Toby also didn’t seem to be hungry when the others were.
Finally, the food bag had been tied shut with one of Mr. Hawke’s ‘super mystery knots’, and Mr. Hawke didn’t notice anything wrong until he’d opened the bag, so the cookie thief must have been able to tie a knot like Mr Hawke’s. Daniel already knew that Toby was the only camper in the group who was good at knot-tying.