Short post: when you agree to the terms and conditions of Google sponsored WiFi (e.g. at Starbucks) your DNS resolution settings are updated to point to Google’s DNS servers. While this does result in hands-off protection from malicious websites it also enables Google to track your browsing habits and gather a large representative sample of the habits of people that use that particular WiFi network.
In Linux, look at your /etc/resolv.conf to determine if your DNS server has changed. Google’s servers are: 126.96.36.199 and 188.8.131.52.
I recommend checking this file each time you connect to a public WiFi network.