Its entiely possible for nems to eat fish. It's not the norm but it does happen. Big reason I got rid of mine, I was always worried he'd eat a fish LOL
How big is your tank? Could he be hiding somewhere?
180g- Mostly BIG fish and some coral. ~80g Nuvo- My coral tank with "happy fish"~ 90g- FOWLR Not the not happy type of fish~ 125g- Freshwater Malawi Cichlids ~10g- Nuvo- The refugees from the Ich of '18