Later last year, Transport for London (TfL) collected wifi data from commuters' phones. This Gizmodo article shines light on a bunch of aspects of the data collection program, including:

• Informing the commuters: TfL let commuters know about their wifi data being collected through posters, and clarified they won't identify individuals or collect their browsing history.
• Route tracking: At the moment, TfL can tell what station you started and ended your journey at based on your Oyster card - but it can’t tell how you got between two locations. By using Wi-Fi data, merged with aggregated Oyster and Contactless ticketing data TfL has a far richer data source to ensure optimal and evidence based decision making for a wide range of planning decisions.
• In-Station tracking: It isn’t just travel across the whole network that can be tracked by wifi. It’s even possible to track your location within an individual station - presumably by working out which access point that you’re closest to - this can be used to analyze crowding, to staff stations optimally, and to monitor how long passengers have been stuck on trains or held outside of stations - and offer refunds!
• Advertising Potential: Being able to estimate the footfall in different parts of each station - and even roughly how long you’ll be staring at each advert - means that they can offer differential pricing depending on how good each advertising slot is.

Check out the article for all the details!