Nifty little tip...

[PoorOldSpike]

I brought it up to level the playing field

Yep nothing wrong with that, in fact people tell me they learnt things they never knew before from reading my tac tips.
I didn't know about this 'blank terrain pane means dead unit' thing before seeing this thread, but now that I do know, i'll be using it in games myself..
I wouldn't call it a 'bug' though, more of a 'quirk' and I daresay it has a parallel in real-life to some extent, because when a RL unit is killed there may be little telltale signs that its really dead like blood trickling out of the woods, or hideous screams etc.
A blank terrain pane therefore abstractedly mimics that..
Here's a test setup I just did to explain the tip better to noobs -

The German tanks have been shooting at 4 x 45mmATG's in the woods.
1 - Jerry sees one gun icon and a row of stars (some are loose crews and inf)
2 - He clicks on each of the stars and this is what the gun terrain panes reveal, 2 are alive (live panes), and 2 are dead (blank panes)
So he needn't worry about area-firing at the 2 dead gun stars..:)
3 - This endtest 'all is revealed' inset shot confirms that two guns are live and two are dead, just as their panes said.