If I put up a fence....can I keep it?

NOT legally!!!

But, the owners must fight for it.
Placing a fence and building a shack is a favorite tactic. If no one protests, they try to keep it. Usually through a labor dispute.
A couple came down last winter to find their lot fenced and for sale. The sb realty office said--opps--sorry about that.

Watch out for your friend's and neighbor's property!!!!

If you see a fence going in, call them!!!!!!

Make sure they are putting in the fence.

May 6th there was another thwarted effort. This time they messed with a Mexican National who came to San Carlos and tore the fence down as she stood there yelling and screaming that she had 'bought' the property from the owner.
He had never seen her before!