I actually tested my Nest doorbell camera last night-- with my porch light off and approaching from the side of the house, I couldn't see the pinpoint green light that indicates a camera is there until I was within 2 feet of the camera, which is also when the doorbell light indicator that is much bigger than the pinpoint light comes on. It still registered "person seen" and sent me a notification of myself, but I couldn't tell there was a camera there. I don't think he saw the camera the first time.
Also if her old doorbell is still connected to the 40v that powers doorbells, it may light up at night, so he may have seen that and thought 'no doorbell camera.'