I woke up to a hot Macbook Pro with just 34% battery after fully charging it, unplugging it, closing it, and putting it in a briefcase the night before. It was so hot it was uncomfortable to hold, and the fan was working overtime. I've followed the same routine hundreds of times before with no similar problem.
Activity Monitor showed Chrome to be the main culprit, and "prevent sleep" was "yes" for Chrome. I had a dozen tabs open, and closed them one at a time to see what changed the status.
In my case, the problematic website was Forbes.com. I was able to toggle the prevent sleep mode by opening and closing a tab with a typical Forbes article. This site has a heavy ad load, auto-play videos, etc. I'm guessing something in their code cycles new videos on the un-refreshed page and is preventing sleep. One time, their video player failed to load for some reason and the prevent sleep indicator never went to "yes." Not definitive, but suggestive. I'm sure other sites with aggressive ad display methods might have the same effect.