“None of the above” in Image Classification?

https://github.com/experiencor/keras-yolo2
https://arxiv.org/abs/1610.02136
https://machinelearningmastery.com/multi-class-classification-tutorial-keras-deep-learning-library/
https://github.com/MuhammedBuyukkinaci/TensorFlow-Multiclass-Image-Classification-using-CNN-s
https://stackoverflow.com/questions/43578715/how-best-to-deal-with-none-of-the-above-in-image-classification