Let me just weigh in on this because this is something I do know for certain. Those mysterious boxes are dug up by dogs, mostly strays, from dirt piles that appear around the neighborhoods (the dirt piles themselves disappear after a box is dug up, but reappear after some time has passed). Some boxes may also come from pet bushes. They do not "spawn" like seashells do in Sulani. Those boxes are never deleted by the game (unlike piles of feathers or trash) and since pets tend to drop them in designated spots, they can pile up. There may be many of them, but definitely not infinitely many.
Those boxes have always been a part of Brindleton Bay ever since Cats & Dogs was released.
I understand, just thought that it was weird since it looks very odd and gives the player a very OP source for collectibles. I have not played in the area for a long time before so it might have been a thing for a long time. Maybe it’s just a personal preference but I dislike the way they look lying in strange circles all over the place, therefore I thought that it must be a bug. But yeah, as long as this does not affect my game’s performance or the loading screens then it’s fine I guess.
