Or, wow, it's crazy how much Google knows about me.
(I'm writing this in part because I tried to Google variations of "how to find a lost phone with dead battery" and among all the articles in the search results that I clicked on, didn't see anyone write up this technique that I later figured out myself.)
I lost a brand new Android phone recently inside my own home. I knew it was inside my home because I remembered using it after I got home. Normally in that situation I would call it or use Google's Find Device feature to make the phone ring, but the phone's battery had died before I realized that it was lost. I spent a couple of hours looking for it in all the obvious places, like behind sofa cushions, in the garbage can, everywhere in the house I had been (or might have been but forgot) since I last used the phone, but failed to find it. Find Device gives the last known location of a phone, but the error circle encompassed the whole house so that wasn't very helpful.
Eventually I decided to download all of the data Google has on the history of the phone's location using a feature called Takeout, just in case it offered any further clues, and that's when I realized that in Google's databases there's an entry for every time one of my phones moves even a little bit, with a timestamp, location estimate, velocity estimate, and a guess of whether I was walking, running, biking, in a car, etc. (presumably inferred using the phone's accelerometer). I converted the data from JSON to CSV and then looked at the last time the phone moved. (BTW the timestamps are in UTC time, which confused me for a while until I realized that.) Then I looked at my Google Activity History to find out what I was doing at that time. The Activity History told me I did a particular Google search around that time, and I remembered I was sitting in a particular armchair while doing that search. (Google Activity History also has a whole bunch of other information, like every time I launch an app on my phone, or visit a website using Chrome.)
Of course I had searched that armchair already but this knowledge incentivized me to do a much more thorough search, and it turned out that the phone had fallen down a very narrow crack in the armchair to a pocket underneath. I ended up retrieving the phone by cutting open the fabric covering the bottom of the armchair.