The days of an ad-free Gmail experience on Android may be coming to an end. According to Android Police, the latest version of Google's email app contains references to built-in advertisements. The uncovered code hints that users will be able to save ads that catch their interest as messages, but little else is known about the company's approach to mobile ads — or how intrusive they'll be. As part of Google's broad overhaul of Gmail back in May, it introduced a new Promotions tab that displays ads closely resembling regular inbox messages. At the time, Google insisted that most users would expect to find ads in a specialized Promotions section. Many users weren't happy with the change, even though it often results in fewer ads for those who ignore Promotions. We'd expect to see Google take a similarly careful and mindful approach to ads in Gmail for Android, whenever they finally appear. We've reached out to the company for more details.