In my case, I've discovered that Devonthink (document/notes management app) is responsible. I've been meaning to file a bug report about it.
I'm surprised that Apple's power management doesn't have an alert for this. Surely an app that causes my Mac to become glowing hot while sitting in my backpack, not to mention slowly running out of battery, is a pretty important thing to intercept. Meanwhile, I keep being asked if Chrome should be allowed to find devices on my network, which doesn't seem nearly as important.
pmset -g assertions
in the shell will also tell you which processes are preventing sleep, and it'll tell you the exact power assertions that are being held.(`pmset` has some other undocumented commands, you can discovery some of these in its source code Apple releases. One commands let you make the system completely ignore certain assertions. If you disable the "UserIsActive" assertion though you might struggle to wake it up)