In 1930 he became a member of Rote Hilfe ("Red Aid"), a workers' welfare organisation widely (and correctly) believed to have close connections with the Communist Party and with Moscow.
On his release from "protective custody" he worked as a link-man for the Communist-backed Kampfgemeinschaft für Rote Sporteinheit (KG – loosely "Association for fighting for red sporting unity"), liaising with the Berlin city authorities.
From there he returned to Amsterdam from where, till October 1938, he worked as an instructor on behalf of the party's "foreign leadership" for the economically important Ruhr region of Germany.
[1] This involved frequent visits across the border into Germany in order to conduct secret meetings with on-the-spot contacts in major cities such as Dortmund, Essen and Oberhausen.
[1] Seng had good business connections in places such as Essen, Duisburg, Bottrop, Wuppertal and in the Bergisches Land (hill country to the east of the industrial belt) on whom he could call for relevant specialist skills.
Using cover names such as "August" and "Kurt", and working alongside Alfons Kaps, Wilhelm Knöchel and Alfred Kowalke, he produced illegal literature under titles such as "Ruhr-Echo", "Illegaler SA-Mann" and "Friedenskämpfer" ("Fighters for Peace").