I believe he got out because the knot was kinda loose. Being a little fella, he must have slipped through the loose knot and out of the bag. At least the top of the bag was tucked down toward the bottom of the box.

And I know that black milks are sweet...but Ari wanted a SOLID black snake. At least he changed his mind about Indigos.

Darth is currently exploring his tub, looking for ways out most likely.