All through the middle ages, European fishermen from France, Portugal, Netherlands etc would follow the North Atlantic currents across the ocean and they fished on the Grand Banks, always heading back before the winter. They had seasonal communities on the coasts, but were never able to establish permanent villages and were often in danger from Indian tribes. The existence of North America was no secret.

However it was only once sailing ships like the Caravel were invented, that European nations were immediately able to push forward and consistently resupply and repopulate their dominions overseas. What followed then was them basically seizing what they believed was already fundamentally their god-given land to own. For example, the French fished in Canada and thus believed Canada was already theirs. The Spanish fished in the Caribbean and thus claimed they owned all that.