If you walk through a wifi area and have wifi enabled on your phone, the system can track a certain amount about your presence and movement. They could have that data for Belfast’s city council run wifi networks, which are on around 70 buildings – so what can be done with it? If they had enough compelling use cases they could partner with other organisations to grow the data set.
That data includes things like the device MAC address, the SSID of the network you’ve connected to, and so on.
When you login, you give consent for that data to be collected and used. You don’t if you haven’t connected. Most mobile phones announce their presence to find wifi hotspots.