The cat is alive. Nothing exists beyond perception, or at the very least, you can't know that it does. Therefore, the death of the cat cannot occur without your knowledge. The cat is alive until you open the box. If it is still alive, then it was alive the whole time. If it's dead, then it died upon your perceiving it's death.
If a friend of mine dies, and I never hear about it, I will speak of him in the present tense, and be correct, for he is alive for all intents and purposes, until I find out he has died.