Unidentifiable Bluetooth device on macOS

I have a always reappearing bluetooth device on my iMac (macOS 11.5.2). It just shows as a MAC address and is not connected. This is the System Report output.

f3-8f-f4-57-45-c2:
  Address:  F3-8F-F4-57-45-C2
  Major Type:   Miscellaneous
  Minor Type:   Unknown
  Services: 
  Paired:   Yes
  Configured:   Yes
  Connected:    No
  Bluetooth Core Spec:  1.0b
  Class of Device:  0x00 0x00 0x0000

I already tried to remove the device, reset all connected Apple devices and reset the Bluetooth module. Nothing worked, the device always shows up after a few minutes.

I don't see this device on any other Apple device like my MacBook and iPad. So I don't think, it is not something iCloud or HomeKit related.

How can I identify what this device is and how can I remove it?

How much human-parseable information a Bluetooth device broadcasts is entirely up to the device's implementors, so there is no guarantee that you'll be able to find an easy answer. However, there are a couple of things you can do to heuristically make a guess.

1. First, if the transmitter is a standalone device, you can try to find its approximate physical location in your room by running a realtime RSSI display tool (e.g., Bluetooth Inspector) and playing "warmer / colder" with your iMac: When RSSI goes up (becomes less negative) you know you're getting closer, and when it goes down (negative value increases) you are farther away. You will need a long extension cord and good upper arm strength to do this effectively.
2. You can use Apple's PacketLogger, part of their Additional Tools for Xcode package at the Apple Developer downloads page, to filter for data packets coming from the MAC address you mentioned and inspect them. It is more than likely that the packets won't contain plaintext ASCII and so will probably not directly give away what this device is, but the combination of packet timing/intervals, packet length, data variance between packets, and other metadata might start giving you some clues on what this thing is. Truly trying this is going to require at least hour of patience and data manipulation.