Our 16 year old female cat Blackie did this. She started walking into doors a few days earlier so she might have been confused or poorly. Was going to make an appointment at the vets a few days earlier and in the meantime keep her in all the time. However she was crying to be let out one night and I felt cruel forcing her to stay in against her will. The next day she was missing. Called her all day and nothing. After two nights of her not coming home we were worried as it was so unlike her. We put up posters everywhere on lampposts and trees and inside shop windows, with her photo on, a brief description , the offer of a small reward [ £20. It was literally all we could afford between me and my son at that time] and our phone number. Walked around everywhere every day.Nothing. My son opened the front door on the 10th morning of her being missing and she was sitting there !! We were so relieved and happy to see her. She was a bit thinner, but not massively so. No idea where she had been. She didn't have a collar on. But somehow she just wasn't the same. She didn't want to stay indoors, even at night. Usually she liked sleeping on my bed with me. But she seemed distant, as if she didn't recognise us. When she was in she kept weeing and pooing behind the TV, which is something she had never done her whole life. I didn't mind and it was no big deal if she couldn't help it, and we got her a litter tray. But she had never in her life used one and had always gone outside, so used to do her business next to it. This carried on for 17 days, from when she returned. I went into the garden one evening, and she was lying down, barely moving, directly over the spot where a little kitten of 6 weeks, not related to her, died 9 or 10 months earlier. I carried her inside. She tried to stand up and her legs were criss crossed and she couldn't walk in a straight line. We knew the end was coming. I picked her up to try and cuddle her and she cried out as if in pain. So I laid her down on the living room floor. Kept trying to give her small syringes full of water to drink but it just dribbled straight out of her mouth. The she arched her head right back, it was almost touching her back, and let out what I can only describe as a death rattle. I've certainly never heard any sound like it before. Then she went into a kind of coma; she was just staring straight ahead, lying down.We were stroking her and comforting her and she just stopped breathing about 10 minutes later. All of this was on a Saturday night and it was pointless trying to get her to the vets as we knew there was nothing more that could be done for her. Me and my son cried buckets, of course. We lay her on a cushion on the sofa and as it was getting dark we left her there for then night and buried her in the front garden, near the kitten the following morning. I've got it all to go through again with another two. [ The mother and sister of the little black kitten]. I hope my story hasn't upset you and that the outcome is very different for you and your cat. Feel free to message me if you like.x