A local Portland station drew attention to Nordstrom piloting a program to track shoppers in-store using unique identifiers from their smart-phone. The data in question is the unique hardware identifier of the wireless network adapter present in most smartphones. The Nordstrom scenario is an example of the easy to recover original MAC addresses from hashed versions, has the same problem. To prevent linking across different datasets based on a unique identifier such as MAC address, everyone has to adopt hashing.”]
Source: https://randomoracle.wordpress.com/2013/02/17/hashing-does-not-magically-anonymize-data/