The TL;DR - the way the Grindr system is set up lets anyone submit a latitude and longitude and get back a list of the nearest fifty profiles. If you do this three times from three nearby locations, you can figure out exactly where everyone is. This isn’t even particularly hard to implement and requires no sneakiness or advanced technical capabilities on the part of the sender.
Anyway, this means that if you have Grindr and you have your distance shown, anyone can figure out exactly where you are. Grindr could prevent this (or at least make it more difficult) by only responding to requests from the app (rather than any arbitrary computer anywhere) but for now they’re choosing not to. So if there’s anyone out there who you’d really prefer couldn’t find you, maybe best to disable distance if you’re still using Grindr.