It doesnt mean that only your router tried to reach that address. Could be some device in another subnet for example. The timeout is expected since that device is not reachable. It was just to show you that the entry appears in dhcp-server leases with only "D"-Flag and without mac-address...
The device is not in your network... Some other device has tested whether this ip is active or not. It seems to be not active. If you want to know which device asked for that ip you need to log arp messages on your L2 network and search them when somthing similar happens again..
It happens continuously. Apparently some device is looped.
Will you advise me how to do this?
I don't know what to log. I want to know MAC address of this device -then I will be able to find a vendor and thereby reduce the list of possible devices narrowed down, for example, to printers or mobile phones