A possible way of introducing relative spotting (although I don't think It is the way BTS will go) is to ally it to a tighter and, IMO,a more realistic C&C structure.
Currently, for instance, a platoon can have its constituent squads spread across the width of the battlefield, divided into half-squads and thus used in a totally ahistoric manner to provide instant recon. of the entire area. Even if none of the squads are in C&C with the platoon leader, the worst penalty is a command delay to the squads in question; almost complete control is still, unrealistically, maintained.
When units start to "loose it" under fire etc. the player, in fact starts to lose control anyway. Units can be unpredictable when pinned and when they panic or break all control is, at least temporarily, lost to the player.
Why should this not be extended to C&C? Those units not in C&C range cannot be given orders (and will do what comes naturally given the situation i.e. sit tight and shoot back if attacked)and cannot,therefore,report units that only they spot. Those units would remain totally unseen (to all other units other than the spotter)until spotted by another unit etc. At that point the only units that could target the enemy would be dictated, as has been suggested, by the TAC AI.
Just a thought>