Android: If you suspect your roommate or anyone else has been reading your private messages on your phone, then PeeperPeeper can help catch them red-handed. The app creates fake shortcuts for popular messaging apps and takes a photo when someone opens them.
With PeeperPeeper, you can create fake shortcuts for Line, Facebook, Facebook Messenger, Whatsapp and WeChat. Put them on your homescreen and the next time a nosey busybody opens them, the fake app will show the splash screen and then crash , displaying the fake message "Internet connection error". Meanwhile, it snaps a photo of the person using the front camera of the phone. You can later review all the photos and the apps that people tried to open in PeeperPeeper.
It's a fairly basic app and has its limitations, such as mislabelling the Facebook Messenger "Messager". But while a low-tech solution, it can get the job done.
PeeperPeeper (Free) [Google Play Store]