[Imp]

Never had an issue with emplaced AAA guns not firing though dont use very frequently.
I would not want a bunker to open up against air giving away its position unless it was virtually on top of it.

I find dedicated AAA a necessity in WWll as planes can easily make 8-12 passes if you dont wing them, thats way to much info to give away.
As Mobhack mobile even if by truck, guess flight path & have them stationary near it when the attack occurs, rather get good shots after the attack than try & intercept before it occurs. If lucky enough to have standard halftracks armed trucks they can try putting the attacker off & hoping for a lucky hit.
As late war Germans if I have one expensive bit of kit with a big gun & superior range finder fire control etc all my AAA will start round it. Its going to be one of the first units to fire as it can pick off at range. That means its going to be a plane magnet & they are most likely the only thing thats a danger to it, sometimes its hard to resist the odd German toy.