ethernal

You're being a bit paranoid. First and foremost this would be illegal. In all scenarios, it doesn't make sense (the only possible exception is investigative journalist):

• Police would either work directly with the rental car company if they had a warrant, or if they did not have a warrant, this would not be admissible in court
• Licensed private investigators would likely never do this because it is illegal. If they did collect this evidence with the goal of using it in a legal setting, it would not be admissible in court.
• If the CIA or similarly equipped intelligence agency is trying to spy on you, they are going to be able to do that regardless of whether your name shows up a rental board
• An investigative journalist would at best have a civil suit on their hands if they released this information, potentially criminal depending on the jurisdiction and local laws. That said, it is possible it would achieve their objectives - but at a high personal and potentially professional cost.
• Corporate espionage - while it does happen - probably doesn't happen by spying on rental cars. This is doubly true in the era of the internet and electronically connected devices that are easily hacked.
• Your wife's divorce lawyer would never do this. Both because it is illegal, would not be admissible in court, and would likely get him/her disbarred if it was clear that they are the ones that requested this happen.

Just to make it clear, putting a recording device in the car without the rental company and renter's consent is no different than placing a video camera in a hotel room. These are not trivial crimes.

There are valid, reasonable reasons to be worried about hotels posting guest names and rental cars posting names on a board. None of the reasons you cited are reasonable concerns.