TRInvictus
TRInvictus Ok, the app has all notifications off, but it does vibrate and play sound, even they're all off (permission not granted).
I am not an expert, but if you think of a totally different app (like GrayJay/Tubular/Youtube), it does not need any permissions to play sounds (media volume should be set, tho).
It could be that the app receives a trigger to open and then just plays the alarm to let you find the phone.
Also, usually apps like Find my Phone require special permissions.
One of those is device admin, that you can find in Settings -> Security & privacy -> More security & privacy -> Device admin apps.
This special permission allows the app to reset the password, force-lock the device, wipe the data, disable the camera and other things, maybe it can also override volume settings (It's just speculation, I'd like someone to confirm/disprove).
Furthermore, it's worth a try to see if your app asked for Modify system settings (which would explain why it can override sound settings) and/or the alarm permissions (which would use the alarm volume settings instead of media).