Pre-WWDC19 rumors indicated that Apple could launch an accessory (codenamed B389) that, along with the new Search My App (Find My; the result of merging Search My iPhone with Search My Friends), could be used to basically track anything.
Well, the opening keynote of WWDC19 has passed and nothing about the accessory has been commented. However, references have appeared in iOS 13 codes that point, yes, to the release of this new product.
The first beta of iOS 13 comes with a bundle of assets For a device referenced as Tag1,1 the discovery was made by Steve Moser. Developer Guilherme Rambo explained that this kind of package used to pair devices by proximity, just as AirPods and HomePod can be paired with a user's device.
Another asset found also gives us an idea of what this device will look like. We may be talking about a model used for testing which has been forgotten in the system, but it corresponds with the device descriptions (provided by people involved in its development) received by the 9to5Mac.
In short: this new accessory has everything to be really a tag for the purpose of being placed on any item / product; after that, the user then pairs the tag (basically as with AirPods today) in the Search My app and links the tagged product / tag to your Apple ID, as it does today with iPhones, iPads, Macs, Apple Watches, etc.
Once this is done, users may receive notifications when they depart from the tag, preventing them from forgetting the item somewhere. This new accessory is most likely to be announced in September at the special event that Apple traditionally presents to introduce new iPhones and Apple Watches.