> They don't need probable cause to search his vehicle if he had a warrant.

Incorrect. Warrants have to be specific. An arrest warrant is not a general search warrant (in fact, general search warrants aren't legal). You don't lose your rights because a cop is arresting you.

Now, what can happen (and probably would happen) is the cops can take a drug sniffing dog to the car when it goes to impound and while the owner isn't there and claim "Oh, the dog signaled on the car so we searched it".

It's called an "Inventory Search": https://www.fletc.gov/sites/default/files/imported_files/tra...

Cops have used the "odor of marijuana" for years as probable cause to search vehicles. AFAIK the courts have generally supported it.