When does stealing stop being stealing?

My girlfriend and i had this discussion some time ago.
We were in the mountains on her island when we saw this desolate kalivi (small cottage).
She said that it would be a nice idea if we would once live in that one.
Since it was pretty ruined she figured it probably wasn’t owned by someone anymore.
I said that as long you don’t know if that is the case you can not just assume it and take it as your own and live in it.
What is your take on this?
Is a bicycle that you see standing abandoned somewhere for three years for you to take it?
Can you take home with you that nice vase that you dug up and is possibly 1500 years old?