whenever a player enters your network bubble FPS will drop from say a solid 60 - 8 for a couple of seconds then pop back up to 60 once the other player is in your bubble.
I've tested this with friends using Vybor gas station & Veresnik mound as markers(1km), once the other player gets to the mound my FPS will drop for a couple of seconds then go back to normal. This happens every time I encounter another player and can be used to predict players nearby.