It's expensive to keep a car in a big city, let alone a national capital where there are a tonne of foreign embassies and governmental agencies. This tells me either the apartment is temporary, he plans on storing the car at the in-laws or possibly selling it. Since he claims to still have access to the bunker per arrangement with his wife, the likely outcome is he is storing it there until he either moves back in or gets a more permanent place.
But Zagreb is an interesting choice as it is an hour away and he doesn't need to be there for work. Why not get a place in Sisak, population 47,000? It's not exactly metropolitan but it's not in the middle of a forest either. And it's minutes from the village his daughters live in.
Him living in Zagreb and seeing how active he is on FB in meeting people is a sign of what the "separation" is for him. Someone clearly working on saving his marriage would have chosen a place a lot more convenient to visit his wife and kids, not to mention his work at the bunker is there too. Zagreb only makes sense if he wants to meet people.