A great question your’re asking! There are indeed several different types of nature reserves. Some are regulated under federal law, others under municipal or cantonal law. The RPAS map only represents the federal situation, not the cantonal or municipal situation.
federal nature reserves
On a federal level there are three types of nature reserves: „Jagdbanngebiete“, „Wasser- und Zugvogelschutzgebiete“ and the „Nationalpark“. In all of those nature reserves flying is strictly prohibited and violations are sanctioned severely. Those nature reserves are marked yellow on the RPAS maps.
cantonal and municipal nature reserves
Cantons (and to a certain degree also municipalities) have the right to further regulate the usage of drones in order to protect the environment. This of course includes the possibility of banning drones in nature reserves other than the three types mentioned above. Those additional restrictions would not be visible on the RPAS map. However, those additionaly cantonal restrictions are extremely rare. The canton of zürich does not have any lawful cantonal or municipal regulations for drones in place as of right now.
Therefore, if you encounter a nature reserve that is not marked yellow on the RPAS map, flying your drone is legal (in the canton of zürich).