When we "see" an enemy unit, all of our units see it too, and if the enemy is within range, our units will fire upon it. This can create some crazy exchanges between units that are out of each other's Line-of-Sight shooting back and forth at one another after something initially triggers the exchange.
It can also create situations where you absolutely don't want your high value units to fire at low value targets. I drew a picture to try to explain:

Say you're commanding the Russians and you just backed your T-34 out of an engagement with a German Panther. You're down to low health, one more shot from the Panther and you're dead. You've gotten out of his L-O-S, but you're not yet out of his range. To your right, there is an infantry engagement -- some Conscripts are fighting some Pioneers. The Pioneers are within range of your main cannon, but you are not within their L-O-S. The Pioneers cannot see the T-34.
If your T-34 fires at the Pioneers, it will reveal itself and the Panther will kill it. This is why I'd like to see a hold-fire button on ALL vehicles in COH2. This situation happens even more frequently in Blizzards when all units Sight range is drastically decreased.