Dead animals

Parents have three cats. They are allowed outside through a cat door. They do very well and are well taken care of. Their home is nice and taken care of as well. They keep bringing in dead animals though or injured animals that crawl or fly somewhere and die. My parents entire house smells like a rotting carcass right now and I loathe it. In an effort to help them I’ve literally looked EVERYWHERE trying to get rid of it and I cannot find it anywhere. The kitchen smells gag worthy from it. My kids are here with me While my husband is helping my father renovate the garage, so we have been here more often than usual through the week and it just keeps happening. They keep bringing in animals and I either get rid of the live animal or the dead animal or go hunting for something hidden. This smell though won’t go away because there’s a dead animal in this house and I don’t know what to do!!! We can’t find it anywhere!! Have literally moved the fridge and anything and everything throughout the main living areas where the smell is coming from. HELP!!! 😩🤢

Edit- My parents have tried bell collars but they are all “breakaway collars” so the cats will not get stuck on something when their out running and they always come home without the collars on. So they’ve given up using those :-/ Also The smell is literally only found in the kitchen and living room. They keep all of the bedroom doors and everything shut so the cats cannot get in them during the day and none of the rooms smell. It’s just the kitchen and living room and we have looked everywhere. :(