I've lived in this house for 39 years, and never had a rat problem. Then, my mother-in-law, Mom, moved in so my wife can care for her, as she's too sick to care for herself. Anyway, Mom has a 'special' gift. She attracts critters. We had to bring in an exterminator and have bait traps put up to keep the frogs, mice, rats, and snakes out of her house. The smell started a little while ago. We thought it might be sewage from a leaky drain pipe, and I crawled underneath yesterday. Dry as a bone. My neighbor (Mike) is a contractor, and remodeled the guest bathroom. He couldn't quite identify it yesterday, but I opened a closet for the first time in a long time today, and it reeked. When I told him, he knew immediately that it was a dead rat in the attic. He showed me where the rat got in. I guess he just couldn't resist Mom's allure. I crawled into the attic for a look-see this afternoon. I was hoping I could find it easily. No joy. I have batts of R-30 insulation over the entire attic, and special heat reflecting film is over that. In addition, we had an attic mounted HVAC unit installed going on 5 years ago, and the ducts are barely high enough to crawl under; plus there is plywood nailed to the rafters for mounting the HVAC unit. I spent an hour up there, and did some good - I re-spread some of the film the HVAC crew had thrown back, but didn't find anything dead. While I was up there, I was wondering if there is a sniffer or something I could use to find it? I should mention I rarely can smell much, and this pre-dates Covid-19 by many years. Thanks.