[clan]

Nope. As you say yourself: Possibly an antenna connection.

Exactly that. When we redid the floors i our apartment we renewed the electrical wiring and added ethernet and coax. The coax was intended for radio/tv.

There is however an overlap of people who have the foresight to add coax cabling also add ethernet a well.

If I was was re-doing today I would add more empty pipes and fiber.

Cabling ahead has a very high WAF (Wife Approval Factor) when adding new gear :-)