The insiders said Amazon would most likely use its no-checkout tech at Whole Foods. While the company previously said it didn't have plans to use Amazon Go at its grocery chain (the firm declined to comment on the apparent leak), that's not the same as ruling it out entirely. To some extent, it's been limited by practical challenges more than anything. How would it handle the varying qualities of fresh produce, or track frozen foods hidden behind glass?

There's also the question of customer expectations. If you venture into an Amazon Go store, you're likely an early adopter who knows what to expect. Whole Foods, on the other hand, serves virtually everyone. Amazon may have a tough time persuading warier customers who would rather talk to a cashier than launch an app, even if they know it would take considerably longer. Any expansion to larger stores may require a cultural shift in addition to any technological breakthroughs.