I see that you're concerned about a red line appearing at the top of your iPhone screen, and I'm happy to help. This means that there is an app accessing your iPhone microphone. For example, when you're recording a voice memo and you navigate to the Home screen, you will see this at the top of the screen. To make this go away, you can stop recording, or you can force apps to close that may be using the microphone.
Force an app to close on your iPhone, iPad, or iPod touch
If you're unable to find which app is using the microphone, you can tap on the red line at the top of the screen to return to that app in many cases. If all else fails, you can try restarting the iPhone:
Restart your iPhone, iPad, or iPod touch
Thanks for using Apple Support Communities.
Cheers!