Why would the FIND MY PHONE ALERT go off randomly

If you go into your device, Settings > Tap on your name at the top > Password and Security > Change password. Once you have set a new password you will get a prompt asking if you want to log out any of your other devices, select yes on there. If there is any other device logged in as you in the world, they will have to enter the new password to continue using it.

Ensure you follow Apples guidance when setting a password, do use anything similar to what you were using before.

Scroll half way down this article where it says "On your iPhone, iPad, or iPod Touch with iOS 10.3 or later:

Change your Apple ID password - Apple Support

Change your Apple ID password.

Were you logging into anything at the time? The FMI alert can often be triggered when using Two Factor Authentication on any of your Apple devices.