If they take over you're home while your there it is called a home invasion and in some states there is a castle doctrine. Police or not, you have the right to defend your property as many states consider this kind of invasion to meet the bar for "grave bodily harm" to the owner with or without an actual threat.

The way this works in particular is by taking advantage of squatters rights. These are ostensibly used to protect people from summary eviction and give them a chance to get their lives together before they're kicked out. Usually they're used to help people who can't pay have 30-60-90 days (law depending) to figure something out.

What these people do is break into a home that appears unoccupied. This could be because you're an investor and are screening tenants, you left for vacation, etc. After some number of days (depending on law) the squatter has "rights" to live there and it goes from criminal to civil. Since you did nothing in those days it's assumed you allowed it. It's exploitation by a class of slightly smarter criminal. Exploitation none the less. Yet, there's nothing you can do about it that wouldn't inadvertently effect the people who actually need the laws to make sure they don't come home one afternoon with their stuff on the sidewalk. That's why as a homeowner it is absolutely important you police your own property, make sure it appears occupied, maybe have friends occasionally visit, etc. Even if you have camera evidence it will be assumed you did nothing about it in that timeframe and it will still become civil. Your house will likely be trashed as revenge and suing a bunch of strung out junkies won't get you anywhere.

These laws have also been exploited when good hearted people lend a couch to someone in need. Stay for long enough, and now you have to go to court to evict them.

Really a shitty situation for people. Especially grandma and grandpa renting out a house to supplement their social security.

There is something very simple you can do. If the squatter can't prove a tenancy agreement they're out.

Not necessarily, varies by jurisdiction.

In some jurisdictions, the underlying legal philosophy is "Wait a second... You really didn't know you had human beings living on your property? Every person in this neighborhood has a responsibility to know what's going on in their property. The law is therefore not going to lean heavily into your ownership rights if you've been so irresponsible with your ownership that people can just put down roots without your knowledge."

I wonder if same applies to public property. Let's say someone starts living in court house and stays hidden for long enough. Could they then one day just walk naked in the court room in there and drive people out as it is their house now?

Just curious - do police officers have access to a system that proves home/property ownership?

The can ask the homeowner to show physical proof of title. Not rocket science.

Not unlike car license when pulled over.

My driver's license has my photo and my home address on it.

This would indicate that you reside there, not that you own the location...

I only care about primary residence, any problems that landlords have can be solved by selling all the extra properties they own.

I'm pretty sure the police could just call the town office, at least in my town, and easily verify who owns a home. I would be surprised if the police do not already have direct access to this information.

The problem is that if the criminal is slightly intelligent, they will present a fake lease and/or some mail they received at the address. At that point, it's the court's problem to assess the evidence and figure out if it's a case of squatters or a self-help eviction by a scummy landlord.

Rentals aren’t centrally registered. They just produce a lease signed by someone who says they have control of the house, doesn’t have to be the property owner, doesn’t have to be notarized. So just print something official and say the landlord signed it. The courts eventually figure it out as fraud anyways, but you have six months rent free, and most prosecutors aren’t going after petty fraud, so it’s mostly a risk free crime unless you accidentally occupy a house owned by the mob.