It is the closed-world assumption of a CNN. For example I have trained a CNN to recognize, sedans, jeeps, trucks, suvs and crossovers, and I present an airplane it tries to fit it into of these 5. How do I get a CNN to recognize a "none of these" category.
I have read this paper Bendale et al but this seems to re-architect the Softmax with their Openmax. Are there any "hack" approaches that do the trick?