Discussion on answer by Mark Johnson: Schengen Visa as Seafarer

The terugkeervisum is entirely irrelevant. It is for residents of the Netherlands who need a visa because of unusual circumstances. See ind.nl/Paginas/Afspraak-aanvragen-terugkeervisum.aspx (in English:ind.nl/en/pages/appointment-for-a-return-visa.aspx).

Since OP clearly states that he has a type C multiple entry visa, most if not all of this answer is likely irrelevant. Unless limitations are clearly stated in the remarks on the visa, I do not see any obvious reasons why that visa can't be used for a regular, non-work-related visit.