Maertens was the son of what his wife, Carine, described as a hard-working middle-class couple: Gilbert Maertens and Silonne Verhaege. His mother was the daughter of a shipbuilder in Nieuwpoort harbour. She had a grocery and newspaper shop, which delivered newspapers. Gilbert Maertens, the son of a self-employed bill-sticker, was a flamboyant and restless man who was a member of the local council and on the committee of the town football club. He ran a laundry with a staff of four behind his wife's shop.