I do not understand: if the building is empty, then there are no units inside…
Of course there is a big mistake in the rulebook (the same mistake that allows you to destroy an empty buildig at 3+, but if there is a small unit down in the same building you target it at 6+!!!). The fact is simple: until rulebook consider the hidden set up a COVER bonus it will be ignored by he against buildings. Forget about the meaning of the word “hidden” and check the rules. Sadly they are clear, and that’s a real bug to me too, but that’s how it works: hidden set up=bonus cover/he against buildig=ignore bonus cover…
Hidden set up is a COVER modifier (-4 soft COVER/-5 hard COVER). HE ignore cover penalties against building, so:
HE against a unit hidden inside rough ground (hard cover) gets a -5 modifier to hit.
HE against a unit hidden inside a building doesn’t get the to hit penalties for cover (HE shoots directly the wall, not the unit, and the wall is NOT hidden!).
We can agree that rules are raw, but not the other points of view. These are the rules of the 2nd edition, everything else is home rule or personal speculation. Let’s wait for a specific faq or a 3rd edition book! 😀