they can x-ray, dog sniff, and ask you if they can search your vehicle all they want.
what they can't do is intrusively search your vehicle on a whim.
it's true that you lose any reasonable expectation of privacy at border checkpoints or their equivalents (airports/seaports), but you don't lose it completely. there are still things police officers can't do, and what the BP did in this case is one of those. Especially the apparent lie about the dog sniff. Getting evidence off false pretenses like that is uncons utional and everything found under such cir stances should fall under "fruit from the forbidden tree"