How is the water escaping?
There is a general belief that plastics are impermeable, probably because many liquids are stored in plastic containers. This is, however, not correct. Plastics are permeable to water and also to gases. The permeability is low but is not zero and therefore, when very long periods of time are involved, a perceptible change occurs. If absolute impermeability is required, then metals should be used. 

The process involved is diffusion and depends basically on the type of plastic. The ones containing fluorine (Teflon and the like) are the less permeable and the polyesters the most permeable, probably because they are more polar. 



What has happened to the plastic balls is that some water has permeated out and in some cases some air has diffused in.