Radar sneaking is when your radar blip becomes invisible to all enemy players, due to holding still or moving very slowly (such as moving while crouched) for 3 seconds. Your blip will become visible once you jog, jump, or shoot, and will become invisible again once you hold still or move slowly. This status is shown with the sneak indictor at the top right of your radar compass. When it is fully visible, this means that your radar sneaking is active.
There is a new feature however that detects players who are radar sneaking. All infantry classes can, within a certain radius, force sneaking blips to appear on every teammate's radar. For most infantry classes however, the range is very close.
50m sneak detection: Captain, Kapitan, Spy
25m sneak detection: Sergeant, Starshina, and Flamethrower
5m sneak detection: all other infantry classes
Trying to find those pesky engineers sneaking to your ore silo? Grab a captain! Is Tanya hiding out in a building while her C4 charges are counting down? Grab a flamethrower or starshina!
This information is also accessible on the F1 help screen under the new Gameplay section.
Thanks to @ Eggman891 and @ moonsense715 for making sneak detection possible in our games.