What is it called when you record someone without them knowing?

You are not supposed to do so. I am not a lawyer, but I believe that it is illegal. However, if you do record a conversation, I do not know how the police will know that you did it. You would have to tell them "I recorded a conversation and the other person did not know about it. What are you going to do about my crime?" As a practical matter, I think they would say, "Go away, little boy!" (Or Little Girl, as the case may be)