Make the CNN to say “I don't know”

Question

I am currently working on an image classification problem. To ease the implementation I used transfer learning in Keras with Resnet50 as base model.
I have 2 classes, however there are many possibilities that the input image may not belong to both the classes. In such cases the CNN should be able to output as Unknown.
Note : I can not create a separate class for unknown as it can come from any distribution.

I read that the Bayesian neural network would help in such cases.

Could you please help me in understanding how can I implement Bayesian CNN with transfer learning. Any material or reference link would be really helpful.

Thank you