Interesting. What if I own a shop and I want to use this technique to graph the number of people near my shop over time? Would I be in the clear if I hash the MAC addresses?
Operations before storage don't avoid any legal issues around the collection, but they certainly can obviate legal issues with storing the data. For example, if you 'hashed' them with an algorithm that always yield 0, you'd surely be in the clear as far as the storage goes. Probably you'd still be fine if the output of the hash was 1 bit. I don't know whether you'd still be in the clear for more common hash algorithms.
