My house doesn't have a sign 'afixed' outside saying it's private property. But it is. More to the point, you obviously knew you didn't belong there.
And no, I do not make it a habit to just walk in an open door of private property. I knock several times before making such an entry.
So if you knocked several times at your neighbor's door and received no answer, you'd feel free to just walk in and snoop?
Very sad.