Bag Ball

Could observing furtive motions constitute probable cause to search a person or a vehicle?
For example, as a law enforcement officer, I have stopped a vehicle for a traffic violation. While approaching the driver’s window, I observe a passenger in the back seat quickly push an container (e.g. a backpack, brown paper bag, ball of foil) under his seat. Drawing on my knowledge and experience, I believe whatever is in the container being concealed is likely contraband. Do I have probable cause to search the item that was concealed in a suspicious manner? If not, do I need more elements like suspicious odors, behavior, answers to simple questions? (Where are you coming from? Where do you live? Do you know this neighborhood really well? etc.) Does any of this change if it does not involve a vehicle but instead a person on the street who, for example, shoves something quickly into his pants or his jacket when I contact him? I would appreciate input from people who have built cases under similar circumstances.
Yes, the US Supreme Court has ruled that is enough for an officer to perform a search in both cases.
Because vehicles are mobile, and because there is a lesser expectation of privacy on a public road, The Carroll Doctrine (Carroll v. United States) permits a warrantless search of a car when the officer can articulate a reasonable suspicion. Furtive movements would be enough.
As far as the person on the street, you have a similar situation where your training and experience tell you the person is hiding evidence of an illegal activity. This is covered under Terry v. Ohio, also called a “Terry Stop”.
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