Awhile ago, I added the "Find My Device" site to my bookmarks and I'd tested it out a few times. So I started there. And I also used Google Voice to place a voice call, so when it didn't ring in my home, I knew it wasn't here.
The Waymo passenger answered and there was much giggling. She kept saying she didn't know what to do. I said just leave it in the car.
So, knowing it was out of my control, I sent the remote wipe command, and hoped for the best. It turned out, the passenger also used the "Emergency Call" to send a text to my emergency contact. She offered to leave the phone in a pharmacy across town! I don't know how that would've helped.
Anyway, I did recover the phone at the Waymo Depot. It had obeyed the remote-wipe command and it was factory reset, with a full battery. It actually came out better-than-new, as the subsequent updates applied a few nice features.