I'm sort of curious where the law stands on this (I am not a lawyer).

Since it has a license plate on it, it in theory displays some ownership info. Is that enough for me to say, "it's clearly not mine now"? If it didn't, does that give me any right to take something off a public roadway?

Obviously, I know that the letter of the law, and what actually will be enforced, are two different things. Taking something that belongs to CBP would almost definitely be prosecuted in this case, regardless of whether it's legally fair game to do so.

It appears that I can't direct-link to it, but look up case 19S-CR-00528 on public.courts.in.gov - this was a case in which the Supreme Court of Indiana overturned an earlier ruling that removing a GPS monitoring device from your own car, when you weren't aware it was there, was theft.