This actually happened to me a number of years ago. I went to a park during the week which I had passed by on many occasions and was for the most part secluded with a serenity that I much liked. It was just me with a blanket and book. Stripped down to my shorts and just lay there relaxing. Decided at some point to take off my shorts too. I guess I got a bit too relaxed as I fell asleep. After some time had passed I became aware of a wet sensation on my butt cheeks. As I became more awakened and realized I was not dreaming flipped over and there was this elderly lady with her dog standing over me. I just froze. She had this huge grin that turned into a smile. I must have turned several shades of red and covered myself. She walked away with her leashed dog following along.