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A candid picture is when someone is not expecting the picture. They usually are not aware but sometimes they might be doing something and also cannot react to the picture that is being taking. You can also use the word "pic" as a shortened term for picture.
You could also say a surprise picture because you are surprising that person with a picture. The term "candid" is more generally used though.

Photography using a hidden camera or where the photographer is hiding with a camera, is known as spy/concealed/covert photography.
"There was a man hiding in the bushes engaged in covert photography."

Example sentence-
She took a candid photo of her son while he was playing in the pool with his friends.

The word "candid" means in secret or something done without someone's knowledge.
"The private detective's job is to take candid photos of people they are hired to investigate.

When you take a photo of someone who has no idea that they are being photographed, it is called a 'candid' or an 'unposed' photo. Unlike posed photos or portraits, where people are instructed to smile or act in a certain way for a photo, candid photos are meant to capture spontaneous moments or everyday life.

I like spontaneous photos because the people are not posing, like they are in a portrait or selfie. 'A spontaneous photo' is when the photographer takes a photo of someone and that person doesn't know that they are being photographed. So the subject is usually not looking at the camera. 'A surveillance photo' is a photo that a private investigator takes of a person being observed or watched, so usually their subject is not aware that they are being viewed.