If you've received emails from Apple that your Apple ID sign-in has been recognized from a new device or your password has been updated, your Apple ID may be compromised.
Luckily, you can use Apple's account recovery tool to reset your account. You can also prevent this by creating a stronger password and using two-factor authentication.
If you hear strange sounds while on a call or your camera app is opening without you explicitly requesting it, you may have spyware installed on your phone.
Spyware apps will ask for camera and microphone permissions that are out of the ordinary. Don't allow them access and investigate the apps requesting these permissions.
Spy software will often run in the background of your iPhone without your knowledge; however, it will burn through your phone's battery at an irregular rate.
Monitor your battery life for irregular power consumption. Another sign to check for is the physical heat of your iPhone, as spy software can cause your device to overheat.
If your iPhone screen becomes unresponsive, apps become slower, or your device randomly shuts down or reboots, this could be a sign that you have spyware.
To completely remove any spyware, you can do a factory reset by going to Settings > General > Transfer or Reset iPhone, and then "Erase all Contents and Settings."