At my family home, my room was out in the backyard in a separate bungalow so I had quite a bit of privacy there. My Grandfather would walk around the backyard every day in the early afternoon as part of his routine. One day, when I thought I was alone at home, I had the window open and I was masturbating and did not notice that my Grandfather had come home and was walking around the backyard and he saw me masturbating. I quickly shut the window (frosted glass so it was not visible) and hoped that I would not get into trouble.
After 2 days, neither he nor my parents said anything and I did not get into any trouble so I was curious. On the third day, when I came home from school that afternoon, I left the window just open enough for me to close quickly but to allow a view of me standing at my desk while masturbating. I undressed so I was naked and stood at my desk waiting. Soon enough my Grandfather came out and started walking around the backyard and I started masturbating just before he approached my window. As he passed by, he looked in but we did not make eye contact and he continued walking around the backyard. I stopped masturbating and waited until he approached my window again to start masturbating again. Every time he passed my window, he looked in. This excited me to no end, as I knew then that my Grandfather wanted to watch me. After a few rounds of him walking around and my stopping and starting, I started to moan. When I was near to cumming, I saw that he walked faster as he heard me moaning and he then lingered at the window to watch me cum.
After that, it became nearly a daily routine with him walking the backyard and watching each time as he passed by the window. If family members came outside and I had not cum yet, my Grandfather would greet them loudly so to alert me and I would quickly close the window. When they left the backyard, my Grandfather would knock on the window. As we had both learned that I got extra excited from nearly being caught whenever someone came outside, he would stand there watching me masturbating as I always cummed very quickly afterwards.
We never did anything else except for him watching me nearly every day. This lasted for about 8 years until I moved out of home at 22.