A security research firm has revealed an Android exploit which allows a rogue application to record all on-screen activity without the user's permission. This has been fixed in Android 8, but users on older versions are still vulnerable. Fortunately, the exploit works in a way which causes Android to create a notification in the status bar.
https://www.notebookcheck.net/New-Android-security-flaw-allows-screen-recording-without-direct-user-permission.264054.0.html