A local group of security people have been running a weekend project they call Project Lion Cage where they take Chinese cars into a local mine with spectrum analyzers etc. to watch where they send data and so on. This is how the bus was evaluated as well. Tor Indstøy has quite a few posts on his LinkedIn page talking about the work and what they have found.

Press release (Norwegian): https://www.mynewsdesk.com/no/ruter/pressreleases/ruter-tar-...

If you wanted to maliciously get around this you could just listen for background radio signals. When AM radio and GPS vanish, don't transmit.