The boxes look rather like normal litter and, as you mention are located in the south east of the town, just past the forest. If you open them you'll get the curator's picture frame. Is it possible you've completed the quest without realising? One way to check - is there or was there a kite in one of the trees on the north bank of the river? If so, you've already completed it, as that doesn't appear until the boxes quest is finished.