I don't know how true they were,but I have read reports that Leif Erickson or his son led an expedition to the northeastern shore of what is called America today,but were ran off by the local Indians.
I have also read more than one report of the graves of white people have been discovered in what is now called Ohio,and they had Norse stone engravings on the stones covering the bodies.
Stands to reason that ancient sailors have landed more than once on probably every land mass on the planet,due to wind and tide,and then just having to "go with the flow".
They had no idea where they were,so they didn't think it was a big deal and didn't usually report it because they had no idea how to even return there.