A device was removed from my account without my permission or knowledge. What are my options to locate the phone

How can it removed from your account, unless somebody knows your Apple ID and password. Options about finding iPhone is none, contact police, contact wireless carrier so they can "blacklist" phone.

This happened to me too, my phone fell in the pool and quit working so I took it to a repair guy. He couldn’t fix it but my iPhone 7 stayed signed in and I could still see where it was last active ( the phone wasn’t backed up for 2 years) but recently I noticed that I could see iMessages that took place 2 months before said incident. Now my old iPhone isn’t showing up and I’m worried.

I removed a device by mistake going through my account looking to find a backup I needed files from. How do get the device added back? It has all my itunes on it.