"Return to combat area!"... in the middle of the map?

It happened to me too, I can’t say where exactly but in different maps for sure

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.

In the spots you have posted in pictures, those are spawn areas for vehicles.

The measure was added because of griefers who would block it just because they want to ruin other peoples game, or players who would camp it, and use the spawn block mechanic to ensure they took the vehicle for themselves.

Was it a good solution? ... eh.  Would it really have been so hard to simply create a volume, and when the game would spawn a vehicle and check inside the volume, and if the place it would spawn is blocked, try another spot in the volume?  

There ARE spots in the game though that do not have vehicles spawns but you can still end up out of bounds.  

Oh, but a bonus, all weapon emplacements are "vehicles" in the game too.  So if those get destroyed, the location they were at becomes a kill zone until the emplacement returns. 

They can get this done in secret, but can't openly place these kill zone volumes in places exploiters get to to gain an unfair advantage.  

Painknight​ Ah that explains why it was happening to me on B flag of Cairo Bazaar Breakthrough in the final sector. There was a destroyed MG in the spot it was happening in.

It happens when capturing flags in the middle of the map while using the vehicles. There are no vehicle spawn points in those areas. THIS IS A MAJOR BUG.