Unrecognized Bluetooth connection with numeric username

Check if this remains connected even when you walk away to another room with concrete walls or say 1000 feet away.

Try the following that may help remove the mysterious device

• Settings > General > Transfer & Reset iPhone > Reset > Reset Network Settings.
• You may have some third-party apps installed especially for Audio, Video, Airplay etc please delete that.
• You may have some profiles installed by some nearby WiFi Router/Bluetooth accessories.

1. Reset Network Settings: Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings: Only network settings are removed.
   1. When you reset network settings, previously used networks and VPN settings that weren’t installed by a configuration profile or mobile device management (MDM) are removed. Wi-Fi is turned off and then back on, disconnecting you from any network you’re on. The Wi-Fi and Ask to Join Networks settings remain turned on.
   2. To remove VPN settings installed by a configuration profile, go to Settings > General > Profiles & Device Management, select the configuration profile, then tap Remove Profile. This also removes other settings and accounts provided by the profile. See Install or remove configuration profiles on iPhone in this guide.
   3. To remove network settings installed by MDM, go to Settings > General > Profiles & Device Management, select the management, then tap Remove Management. This also removes other settings and certificates provided by MDM. See “Mobile device management (MDM)” in the iOS Deployment Reference.
   4. Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
   5. 
2. If the issue persists, you can try resetting all settings on your iPhone. This will not erase your data but will reset all personalized settings to their default values. Go to Settings > General > Transfer & Reset > Reset > Reset All Settings and confirm the action. Note that you will need to reconfigure your settings afterward. All settings—including network settings, the keyboard dictionary, the Home screen layout, location settings, privacy settings, and Apple Pay cards—are removed or reset to their defaults. No data or media are deleted.
   1. Reset All Settings: Return iPhone settings to their defaults - Apple Support:- Return iPhone settings to their defaults
   2. You can return settings to their defaults without erasing your content.
   3. If you want to save your settings, back up your iPhone before returning them to their defaults. For example, if you’re trying to solve a problem but returning settings to their defaults doesn’t help, you might want to restore your previous settings from a backup.
   4. 

It might be Bluetooth Low Energy Device (BLE) like a smart speaker, door cam, door lock or even another iPhone looking looking for a BLE device.

If the little circle with an I in it, is showing you’re connected. Tap the icon and you can disconnect. If the icon is not present, you’re not connected.