Walmart has been surveilling its checkout registers using a computer vision technology called Missed Scan Detection to identify when items move past the scanner without having been scanned. As reported by Business Insider, the technology has been implemented in more than 1,000 stores across the US over the past two years, and it monitors both self-checkout kiosks and traditional registers managed by human cashiers.

The system runs on cameras that watch as items move across the register. If an unusual activity occurs, such as an item moving into a bag without being scanned, a checkout attendant will be notified to take action. Missed Scan Detection was designed to help reduce theft and other losses, a problem that has cost US retailers up to $47 billion in 2017.

In the two years since the system was deployed, Walmart says it has reduced rates of theft, inventory loss, fraud, and scanning errors. The company, however, did not specify exactly what those rates were, or how much money the tech helped stores save.

Though both Walmart and Amazon have been investing in AI and computer vision technology in their stores, the approaches are very different. While Amazon uses AI to create a cashier-less, seamless shopping experience, Walmart has been implementing technology as a way to boost productivity for its employees. In April, Walmart opened an AI-powered store that uses computer technology to monitor live inventory so employees know exactly when they need to replenish each section. Walmart has also invested in robots to help with cleaning floors.