Okay, I had a chance to look at the SK 2 and SK 4 rules last night concerning the question.
The first thing to note is that per 6.4 any attack on a hidden unit is resolved as area fire, ie. half-fire power, which makes it more difficult to have an actual effect on the unit. To reveal a hidden unit an attack must have an effect on the unit with a minimum of a PTC check. Firing at the hidden unit is not sufficient to reveal it. What this means is, if you have a hidden unit in a hex that may be affected by fire, you are not obligated to inform your opponent of the presence of that unit unless the attack would affect the unit. Of course, in an "empty" hex this is a little more complicated, because if a hit is gained on that hex, a subsequent roll is needed to determine the effects on the HIP unit. There is no obligation to inform ones opponent the nature of the HIP unit until such time as there is an effect. (Someone who knows the rules better--am I correct here?)
With regards to targeting, per 6.10 and 6.11 there are two basic ways to attack a unit--as a target (Infantry Target Type and Vehicle Target Type) and as an hex (Area Target Type). ITT and VTT both use 1/2" acquisition counters, while the ATT uses the 5/8" acquistion counter. In all cases a hit is required to gain an acquisition counter, and their modifiers do not apply to the initial attack. As the ATT hit is on the hex and not the unit, only the hex and NOT the unit is acquired.
With regards to the ITT and VTT, you may only target a unit that is visible on the board. Because you do not see a HIP unit, they may not be attacked with either. And even if you find out per above that the hex contains a HIP unit, you still may not attack it using ITT or VTT because in game terms nothing has changed. Therefore, the only way to attack a HIP unit is with an attack in such a manner that the attack affects the whole hex; in this regard it would be an ATT.
What kind of attacks affect the whole hex? Flamethrowers, DC charges, ordnance (ATT), and OBA (per Decision at Elst). Even if an attack by ATT is a hit, the only acquisition is on that hex, not on the HIP unit. Also, in the case of guns we are actually talking about units, not a unit, ie. crew and gun. An attack that affects the crew may not affect the gun, and though the crew may be revealed the gun is not necessarily revealed.
Well, I am sure there is something wrong in all this, but this is my (current) understanding of rules.