To clarify, in this particular case the connection was stronger than what you state. He bought the SUV from his cousin and the tracking device showed up shortly after he drove his cousin’s wife to Tijuana, I assume with the SUV in question.

While we can debate whether this is sufficient cause for a planted tracking device, the connection between him and his cousin is a lot less casual than what you state.

Fine, then get a judge to issue a warrant.

Exactly I'm pretty sure what gvb's comment states would be enough to convince a judge to issue a warrant.