I actually have an old Google Bluetooth Titan Security Key, but it doesn't seem to work with Smart Lock. It doesn't let me add it as a trusted device despite being successfully paired. I think it might operate on Bluetooth Low Energy.
Seems like NFC might be better than Bluetooth for my purposes since I wouldn't have to worry about battery life.
I don't know what Bluetooth device might work best for this purpose. I believe products like Tile also work on Bluetooth Low Energy.
Any recommendations on NFC tags? Seems like Etsy might be good to get something somewhat customizable and better looking than the blank chips I've seen listed online.
Happy to hear anything you have to share on this topic!