Hi, and welcome to ITKE.
It is easy for any device to have a fixed IP address, just by configuration of the network parameters.
My guess is that someone brought in a device with a fixed IP address, connected it to your network, and that is how you saw this. That device is probably now gone, which is why you can no longer see it. It is also possible that it’s IP address has been changed to one that IS within your range, so an audit of all devices connected to the network would be a good precaution.
There are several software applications that can do this for you, some are free to use, search for ‘Network Audit software’, and you should find something suitable.