I am currently on a solo train journey from Copenhagen to Stockholm, which is about a 5.5 hour ride. I boarded normally, found my assigned seat in my booked carriage, and everything was fine at the start.
About an hour into the trip, the train stopped and an older married couple boarded. The husband sat next to me and the wife sat across the aisle. No one said anything or asked to switch seats.
Around 30 to 45 minutes later, I got up to use the restroom. When I returned, the wife was now sitting in my assigned seat, and my backpack and sweatshirt had been moved to what was previously her seat. There was no explanation offered, no request, and no attempt to switch back. They simply acted as if nothing had happened.
Since there was a language barrier and they did not speak English, I ended up taking the other seat, but I am honestly confused by what just happened.