Droplifting

"Droplifting" is the opposite of shoplifting -- it's bringing an item into a store and leaving it on the shelves. Usually these items have no scan codes, so they confuse the clerks at checkout. Droplifting can be done for the fun of confusing the well-ordered consumer system, it can be done to convey a message (as, by attaching unauthorized stickers to products), it can be done by authors or musicians who want consumers to "discover" their artwork without the hassle of getting a label or publisher first.
http://weburbanist.com/2007/12/26/shopdropping-the-subversive-art-of-reverse-shoplifting/
https://en.wikipedia.org/wiki/The_Droplift_project
http://shopdropping.net/

I have done this at the library though I didn't know there was a name for it.

I had a dog once that was good at droplifting (of a sort).