How does Apple privately find offline devices?

Each time Lassie sees some (unknown) device broadcasting an identifier, she won’t know if it belongs to Timmy: but she can send it up to Apple’s servers along with her own GPS location. The pseudonym approach described above should work well to keep Timmy‘s identity hidden from Lassie, and even from Apple (up until the point that Ruth searches for it.) Ruth can send in Timmy‘s pseudonyms to Apple’s servers, and if Apple finds a match, she can obtain and decrypt the GPS coordinates.

Source: blog.cryptographyengineering.com