Which rules it out for the date of the OP's sighting
Not necessarily. Last return Koper >>> s'Hertogenbosch was 31/8, and the OP referred to "yesterday" on 1/9... it could conceivably have been the northbound service.
I think it is more likely that this was a freight movement of new cars manufactured by
Revoz in Novo Mesto. Revoz is owned by Renault, and the Novo Mesto plant currently manufactures the Renault Twingo and Renault Wind. The plant was most famous for building the Renault 4 up until the eighties.
I don't know for sure if Revoz export cars by rail, but the plant
has a pretty substantial rail facility.
Edit: looking for pics,
I found this from 2007. They look like mk. 5 VW Jettas, which I think could have been built in Germany or Slovakia.