unknown bluetooth device

It is someones device who is near you if it shows up when you open Settings/Bluetooth and it appears under “Other devices”.

Or it is a device that uses BLE (Bluetooth low energy), that “pairs” with an app instead of with the phone. When you install the app it leads you through the process to pair it to the device. Once paired it can only communicate with the specific app it is paired to. It will never be a malicious source; it is always an app that you have installed on your phone to go with a device you have purchased, and that you went through steps to set up. It is typically used for health and medical apps, such as exercise monitors and watches and medical devices such as blood pressure cuffs, scales, blood sugar monitors, etc. And it is very short range; typically only a few feet.