There is significant research supporting the notion that smells we do not even consciously notice, affect our behavior. Throughout the day, our eyes and ears are almost always consciously detecting what is around us. The nose, however, repeatedly enters a sort of state of "rest", where it is simply waiting for some type of discernible odor to enter. Yet, during this time, the olfactory system is still sensitive to scents that are too weak to enter the threshold of consciousness. Very mild scents that we are unconsciously receiving can actually affect our thoughts and behaviors. Although we are not aware of the odor, our brain is still responding to it. For instance, if a person is exposed to an undetectable cleanser smell, they are much more likely to make plans to clean something.
Businesses have caught on to this. From clothing stores to casinos and even air lines, scent diffusion among different industries has risen to become a new standard of doing business. The longer a customer is in a store, the more likely they are to continue purchasing. Subliminal scents can either keep them present or encourage their return. Linked below is an article that further describes the trend: