I do not take a GV, anywhere, ever. I also do not attack GV bases with my aircraft. I very seldom defend a GV base with my aircraft. I do not attack GV's at enemy bases.
If, however, you are an enemy GV at my base, game on, no rules except for what the programming allows.
On the icon thingy, the game graphics do not allow by any stretch of the imagination, you to see nearly as well as in RL. The eye is able to distinguish colors much better, the straight lines that you do not see on a natural object allow the eye to percieve a GV from the air so much better than the game graphis allow that I believe the inclusion of enemy GV icons needs to be in place to better simulate reality.