Google volgt websitebezoeker offline

The analytics tool determines whether a person who clicked on an ad went on to visit a store, using data from Google Maps users on Android and Apple devices who have their location history switched on.

Google says it has mapped "tens of thousands of stores" in the UK in an effort to link its online ads to driving in-store purchases.
The user's unique Google ID links the visit with an ad that has previously been served or clicked.

MAPPING STORES
"Mapping" stores involves ensuring each retailer's Google Maps location is highly accurate, so it can pinpoint when a user actually entered the store (rather than stopping outside for a moment or visiting the store next-door.) 

BRON: http://uk.businessinsider.com/google-already-mapped-tens-of-thousands-of-stores-for-in-store-visits-analytics-2016-4