Sad news, my sweet Neo is gone

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Friday afternoon, around 5 pm, my little Neo went outside like he usually does for a little while. It was very sunny and nice. Well, an hour later we thought we heard him "knock" on the door, but he wasn't there. After a little while of not hearing him at the door, we began to worry. It was darker outside and getting very cold, and he usually comes to the door by now wanting food and to come inside. We call for him several times until we finally have to go to bed. He has done this once before, staying out all night, but we didn't call for him that time. He never stayed away even when we called for him. We didn't get much sleep and when he wasn't around the next morning we decided to go look for him. We live in a small neighborhood, and there is a large area of woods behind the house. We started down the hill to the side yard, where a creek is, heading towards the woods. That's when I almost stepped on him, lying there as if he were sleeping. However, he was obviously not alive. There were no marks on him, no signs of injury, until we lifted him up a little and he was bleeding from his nose and mouth! We looked everywhere for signs of an attack, but found nothing. His head seemed a little swollen, too. We thought maybe he ate something poisonus or even got hit by a car, except he was a good distance from the street and we live right by the cul-de-sac. Does anyone have any clues as to what could have happened? I was very upset, he was such a lively and sweet cat. He was only 5-6 months old. In the picture I added, he is a very young kitten when we first adopted him for the animal shelter. I miss my little buddy, and his sister Charlotte (we adopted a set of kittens) has been looking out the window as if she is looking for him. She is not very playful anymore right now. I guess she is mourning for him, too.