It has disappeared I rebooted it came back and has gone again
This is normal.
The game keeps a few enemies queued up on your device. But if you lose connection with the server this can happen because once your queue is empty it removes the icon. As soon as you make connection again it will reappear (usually I have to exit this screen entirely and come back in).
It's easy to make this happen if you are on your phone/tablet and switch into Airplane mode.