That's true, noticed it myself. But the AI still does not know the type of the unit, I think (more like radar). It happened often that one enemy fighter flew into "visual range" of my planes, but not into attack range. And only then the rest of the enemy fighters began to chose its targets and attack my planes. At least that's my reasoning for this behaviour. But how else to execute the simple "Seek&Destoy" AI team order - as it usually has the range of "999" hexes preset. Maybe that causes the "cheating" of the AI....(?)
Okay now, that's something I noticed, too...
BUT: I presume that the AI can, like the player, see movement of units into and out of camouflaged (high cover) locations if LOS is present. Usually you can not tell what unit was moved, but if you observe the AI turn carefully, you can definitely see the units moving in or ouf of camouflage (of course only if the neighboring hexes don't provide cover, too).
Additionally, as a player, you can spot enemy arty positions in the FOW when they fire and use your "indirect" fire trait to bombard them back. The AI can't do that.
However, I really don't know how to explain your little cheater scen.