Nest Cam - how to find out if someone got access to my camera and controlling it?

Hello! I have an indoor Nest camera (Nest Cam A0005) at home. There is something weird happening with my camera. I had a situation that a person came to my house but I can't see it in the video history of the camera (I have a nest aware subscription). Questions: 1)How can I determine if someone got access to my camera and controlling it? 2)Is it possible to "freeze" image on the camera (make it seem like nothing is going on) while there are movements in the room? 3)What can I do to increase security of my camera (to avoid a situation that someone has an access to it)? 4)Teach me how to temporarily insert a constant image to the camera