If you are a mobile developer, you should take a look at the white papers and presentations that have become available following the recent blackhat Europe 2015.
Of particular interest to Android developers is (In)Security of Backend-as-a-Service (pdf) that shows how hard-coding service credentials for services such as Parse, Cloudmine and Amazon AWS not only puts particular user’s data at risk but the whole platform at risk. The easily-obtained credentials can be used to access huge amounts of sensitive data. The apparent ease of using BaaS has caused mobile developers to cut and paste sample code without thinking of the security implications.
Wondering what else your app might be doing wrong? The AndroBugs framework presentation allows you to scan APKs for known coding mistakes. It decompiles the app and looks for bytecode patterns that signify vulnerabilities in the code. Androbugs has recently become open source on GitHub.
Another way your app data might become visible is if it’s stored in the OS sandbox and subsequently backed up either on a rooted device or via ADB. The presentation on Authenticator Leakage Through Backup Channels on Android (pdf) goes into a lot more detail.
As usual, black hat activities look into the problems but don’t give many ‘white hat’ recommendations how to fix apps. It’s easier and more fun to find problems than it is to provide fixes. If you are looking for some answers then my AndroidSecurity.guru site provides some recommendations.