Rebels?

In my first game as England a rebel army appeared on the border between London and Notthingham. London had 190% loyalty and Nottingham 140% so I’m at a loss where those rebels did spring from. In MTW I had no rebels appear on the map as long as I had loyalty above 100%.

Would having a watchtower in the area of Devon helped to make the entirety of the London province visible and would that have prevented that rebel spawning?

Watchtowers will help you spot the rebels and are a good idea in general, but in Medieval 2 rebels will spawn at random somewhere on your map. They are not hugely common, but they happen without any cause.