If the tub is still snap-locked, are you sure that the kitten released the snake? Ball pythons are strong, and can push up the sides of a latched bin if it doesn't have extra clamps on it to secure it.
I learned the hard way that they're willing to seriously damage themselves to do it, too, when a snake in quarantine forced up the side of a bin to escape, and severely skinned her nose. (She healed up fine). They will put their full strength into it, and if there is any flex in the plastic, that's a way out.