I believe cats consider that the dead bird or mouse they place on the doorstep is their token of appreciation, their gift of thanks, to their owner. Maybe a dog thinks it is giving a present. Must work for them as long as they don't notice the bag being deposited in the nearest waste container. Of course, they probably don't acknowledge it as a garbage can.
I remember reading a suggestion that an alien race looking down on earth might think that it is populated by vehicles that periodically disgorges some of its contents. Is that more likely?