Android smartphones running Lolipop, Marshmallow, and Nougat are vulnerable to an attack that exploits the MediaProjection service to capture the user’s screen and record system audio. Around 77.5% of all Android devices are affected by this vulnerability. Google has patched this vulnerability in the Android OS this fall, with the release of Android Oreo (8.0) Older Android versions remain vulnerable. The attack is not 100% silent, as the screencast icon will appear in a notification bar whenever an attacker would be recording audio or capturing the screen.
Source: https://www.bleepingcomputer.com/news/security/android-bug-lets-attackers-record-audio-and-screen-activity-on-3-of-4-smartphones/