What happens if I am mining in the wild and I bump into someone else's grinder? If I don't break anything or steal anything, am I allowed to use it? Or must I disregard it?
You can technically use them since they are public property. But it would be considered polite to ask the owner first.
I was told by a mod that you can use it, but you have to step aside if the owner/s comes and are going to use it
Like the first 2 comments said, you can but if there are signs with names you should ask the owner first.