There's no such thing as a passive detector.
Sure there is. There is nothing special about a detector. If you can put a whole cat (=a bunch of atoms) in a superposition of quantum states, you can also include the detector (=a bunch of atoms) in that superposition.
It only works if the inside of the box (including the detector) is isolated from the rest of the universe. Then there is a superposition of 2 states: (1) the radioisotope didn't decay, the detector detected nothing, the cat is alive; and (2) the radioisotope decayed, the detector detected the emitted particle and released the neurotoxin, the cat died.
Once you break isolation (i.e. coherence), the rest of the universe entangles (at random) with one of these states, and the other state "collapses".
The big question is: what happens with the collapsed state? Was it absorbed in the other state? Did it entangle with a parallel universe? Did it become disconnected from reality?