An invisible eye may soon be tracking how much milk you're drinking in your own home. Walmart, the world's largest bricks-and-mortar retailer, is developing technology and has applied for a patent that will allow it to track the way products are used once consumers take them home. If implemented, use of the technology would be two-pronged. It will automatically re-order items, such as toothpaste and milk, when the bottles start running low. And it would also track how frequently products are used. According to the research firm CB Insights, that kind of frequency tracking could tell Walmart how often someone wears a pair of tennis shoes. The company could calculate then how quickly the shoes wear down, and make suggestions to consumers about what pair they might consider ordering next through personal promotions. This kind of technology isn't entirely new, though it requires less work than what Walmart's competitors currently offer. Amazon has Dash buttons available for its consumers.