TexasCHLforum.com

My understanding is as follows;

Police (like any member of the public) has a license to approach a door of a home and knock. This assumes that the home does not have a barrier (like a fence with a closed and locked gate). There are special protections afforded to citizens in their own homes.

As for a vehicle on the road, I believe what Steve (srothstein) is trying to convey is that they are treated similar to a suitcase/briefcase. While in public and a police (or any member of the public) is able to collect information based upon what is in "plain view" absent a lawful reason to search. If your car is in a parking lot, there is generally no law preventing someone from peeking in your windows.

I'm not a lawyer, but I do watch a bunch of legal videos on youtube.