Why? I am a Waymo customer. I left it in the car, and so the service has a policy to collect the item and return it to their customer. Some random store where I don't go, all bets are off.
I was able to contact Waymo and discuss the lost item and they were able to recover it because it was left in the car. That is a success. I was also able to retrieve the item from their Depot because their Lost & Found process works. That is a 100% success.
It was unclear in my original comment, but the passenger's offer to leave it at a pharmacy was in a text message to my emergency contact, who didn't reply. I heard about that bit after-the-fact. All I told the passenger, repeatedly, was, "please leave it where you found it in the car" because Waymo would be able to work that out with me, a customer.
A few months ago, I lost the front door key to my apartment. The leasing office was closing in 5 minutes. I needed a loaner key, and I had no ID. I offered to leave my phone (one in the same) as collateral. They were extremely reluctant to be holding on to that instead of an ID. They said they'd make a one-time exception.
Phones are not really things that strangers like to hold on to, in case you've not noticed. If you are admitted to a hospital or jail or something, they get really jumpy about putting your electronics in a locker. Partly having to do with the likelihood of fire or explosion, the volume and value of personal data on those things, and their trackability. The best course of action for a lost device is you leave it where you found it, because that's the owner's best hope of retracing their path and finding it again.
