Apparently its for vehicle spawn protection.
From this steam post, https://store.steampowered.com/news/app/2807960/view/698776157349217018:
"Vehicle Spawn Protection: We've seen some confusion recently regarding "random" out-of-bounds messages triggering in areas of the map that previously were safe to explore. To clarify, this is an intentional design feature to prevent players from blocking or camping active vehicle spawns. If you find yourself staring at an OOB warning in a seemingly normal spot, it's because you are in a vehicle spawn zone. The warning is designed to keep vehicle deployment predictable and fair for both teams."
I think its bugged as it seems to happen in spots away from any vehicle spawns. Even so, its very annoying to be in the middle of a tank fight and then have that keep popping up and sometimes getting me killed.
