WIRED - May 20 - Computer researchers in Kyoto, Japan discovered a stalking method which is cheap, reliable, and works not only with Grindr but other gay dating apps like Hornet and Jack'd. It's known as trilateration. If Grindr or a similar app tells you how far away someone is—even if it doesn't tell you in which direction—you can determine their exact location by combining the distance measurement from three points surrounding them. In the US that's not a problem but in Islamic countries or in Russia, it can be very serious that their information is leaked like that.
In late 2014, Grindr responded to security researchers who pointed out that risk by offering an option to turn off the app's distance-measuring feature, and disabling it by default in countries known to have "a history of violence against the gay community". The lingering issue, however, remains: All three apps still show photos of nearby users in order of proximity.