How To Track A Lost Iphone and Accidently Removed From Device List

Hello.

I had an Iphone 6s Plus which I didnt use more than 2 months as I bought a new one. I left that phone on my table in my office. Now I cant find it and I’m pretty sure that one of my staff took it. I asked everybody but all denied. That phone already in off condition before it lost and I changed my Apple ID password yesterday. I accidently chose “Sign Out From All Device” and now this lost iPhone 6s Plus is no longer in my device list. My question is, can I find my Iphone back or can that thief unlock my phone without passcode or Apple ID? My concern is I have some confedential information in that phone. I rather let that phone goes reset than someone can open it.

Thanks