But also individuals seem to have travelled further, than one would imagine in absence of travel infrastructure. It has been known for long time how the vikings traded with the mediterranean, but recent findings about stonehenge show, how big the sphere of contribution was.
For all societies which had ocean-going ships doing off-costal travels, I would expect them to have occasionally travelled much further, than most history-writing accounts for. Besides the known viking settlements off the American coast, it would actually be quite surprising, if not a few ships had reached the mainland. So I would not be surprised about any artefacts being found in America, pointing at pre-columbian contacts.