A very small percentage of ZCash transactions are private at this time. 38 were relayed in the past 24 hours: https://explorer.zcha.in/statistics/usage
124 karma · joined September 19, 2011
Proof? The key pair signing the transaction proves it originated from this service. HTML5 geolocation data could be incorrect in the same way a Foursquare check-in doesn't prove one is actually at a given venue.