I don't think Google generally opensources _products_ - either it always is open source (Android) or never is (web apps). I can't think of an example where a product was closed source, released as open source, and continually maintained.

Open source at Google generally takes the form of libraries rather than products. Often, that's something that an individual engineer is working on, and it's easier to open source than get the copyright reassigned (since Google by default owns any code you write). There are also libraries that are open sourced for business reasons - e.g. SDKs. You can tell the difference, because most individually-driven libraries contain the copy "Not an official Google product" in the README.