You are not alone in this. It is natural cat behaviour to be nosey at, peruse, sit in, tail chase, or rub noses with any box, bag or other receptacle left within paw reach.
They do this because even the most seemingly sensible and intelligent cat will think there’s something to chase. In a big cardboard box they will poke at the corners or chase their tail. In a smaller bag they will sit and await the return of the beastie that lives there. It’s an unshakeable conviction that somewhere in the vicinity of this bag is prey. And the root of this conviction? Hallucinogenics.
In the manufacturing process of nearly every item made in a factory, there is one common factor. There is a bank of rollers conveying the bag or box to another sector of the warehouse. Normally grey, but occasionally black, these rollers are the same in most factories worldwide since they are so simple to mass-produce. They are made of the same kind of low-friction plastic in standard sizes. These rollers do wear out and need replacing, as all contact parts do. And as they roll the products around the warehouse, slowly wearing thin, they deposit a microscopic amount of residue on all the items they touch.
It is this residue that, although harmless, makes your cat spaced enough to believe that in every unseen corner there is something hiding just out of reach. In extreme cases, she’s pretty much on an acid trip to Invisible Mouseville. But they seem to enjoy it, so there’s no cause for alarm.