My ex is part of organized crime, a biker gang. I’ve posted a question here before about his threat of there being an underground tunnel to my house.

Other than that, he’s also threatened that there are cameras inside my home. He seems to always know small details about what I am doing at home. 

I don’t see any cameras anywhere. Is it possible to have cameras implanted behind the walls? Do cameras that see through walls exist? 

If not, how are cameras usually hidden? Because I do not see cameras anywhere.