The notification was described by Apple as being tied to you, person having an AirTag you don’t know, having a Home location on your iPhone’s “myself” contacts

The notification was described by Apple as being tied to you, person having an AirTag you don’t know, having a Home location on your iPhone’s “myself” contacts card, which presumably then also requires you to have certain System Services permissions to have been left enabled to allow recognition of location by the iPhone in question. Phrasing clumsy, but essentially: If your iPhone doesn’t have a Home icon in Maps for your home, you might not get AirTags tracking protection. Someone should test, or ask Apple to clarify.