How does a Google iOS app save and restore active accounts across app deletion?

On an iOS 8 device, I installed Google Hangouts and upon first launch, it pre-populated an account that was previously associated with a different, and long ago deleted, Google app. Since I had just Reset Advertising Identifier, I assumed it must be the Vendor ID (there was another Google app installed on the device). So, I deleted both (all) Google apps on the device, which *should* reset the Vendor ID. I then re-installed Google Hangouts and launched it. It STILL had the old account active. This is not only mysterious behavior, but is also a pretty serious security and privacy vulnerability. When an app is deleted, and especially when all apps from a vendor are deleted, there should be no active accounts or data left on the device. Any ideas on how Google Hangouts knows about the old account?