IANAL, however, I have spent some time studying the law when it comes to the Fourth Amendment, and technically he is right (though what follows from him gets really hard to follow.) Probable cause only is required when it is unreasonable search and seizure (keep reading before you jump at me, and yes, that is very close to what he said, even though he did a really bad job of explaining it, which I’ll try to do below.)

Reasonable is defined so narrowly by the courts that just about all searches are unreasonable and thus require probable cause and a search warrant. An example of a legal and reasonable search and seizure which wouldn’t require probable cause/search warrant is if a police officer can see, in plain sight and where he/she is lawfully allowed to be, either a prohibited item or evidence of a crime.

Thus, if a police officer is walking down the street, and a small vial of cocaine (a prohibited item) pops out of someone’s pocket and lands on the street, the police officer has every right to seize that vial once determining that it is cocaine. The search and seizure was entirely lawful because it was reasonable.

Same is true if the cop pulls you over and there is a bloody baseball bat or bloody knife in the passenger seat of your car, and it is visible to all who look into the passenger compartment (under a blanket or other object doesn’t count.) A police officer can most certainly seize the evidence at that point, and detain and question you about who’s blood is on the bat or knife. Sure, there may be a perfectly good explanation for it (such as you hit or cut yourself,) and they may choose to let you go, but the seizure, even if temporary, was perfectly legal without probable cause. However, if the police finds a bloody knife in the car, that doesn’t automatically give them the right to open the trunk or go to your house and break in looking for a body, as they’d need probable cause and a search warrant to do that (which likely wouldn’t be hard to obtain fairly quickly given that there is evidence of a crime.)

However, if it is not visible, it would be unreasonable for a cop to see it, and thus they better have probable cause and a search warrant to search for it.

(Won’t get into exigent circumstances, search incident to arrest, or Terry-Patdowns, as those were gifts to law enforcement by the Supreme Court, but they are extremely narrow in their application and we could go on for hours discussing them.)