I would assume I was hallucinating. For a while. Then my existing paranoia and doubt in my own sanity would go completely bonkers. I think I would be too destabilised at that point for anyone to predict my actions.
Although I'd probably call the police after the hallucinating thing was first disproven.
You'd start noticing the smell around day 2, so either I'd find it sooner, or I'd know it was moved. After finding the body, I'd call the fucking cops and pick out a lovely hotel to binge drink at for the next week while I tried to forget what I saw and the police and then the crime scene cleaners did what they needed to do.