If I find a mine that is not part of a town or outpost is it free to mine from?

Because the world is so rich with history of so many players I often stumble upon existing mines when looking for a particular ore. Most of the mines that I've started I have abandoned after getting whatever resources I want from it. If someone stumbled upon one of my own mines I wouldn't expect them not to mine there unless I left a sign asking me not to.

(Assume that there are no signs saying "Private - please don't mine" or "Abandoned".)