I think JR's suspicions are correct - that the Sighting TC mod should be +0 vs. units on a rooftop. The problem is probably just semantics as the "SIGHTING TC DRM" table states:
+3 Target is in building/woods/rubble/orchard (in season)
Units on rooftops are not IN the building. A rule from Red Barricades supports us regarding "roofless factories:"
O5.47 AERIAL: The Sighting TC E7.3) DRM vs a unit in a Roofless Factory Hex is always + 2, not + 3.
This makes sense - a roofless factory can be seen into by aircraft, but still provides some places to hide - much moreso than a rooftop!
And as JR says, it seems odd to use +3 since a rooftop counts as OG for FFMO, rout & interdiction to give it a +3 mod just for sighting. I think a RB update should simply add a passage like O5.47 to section B23.8
As for
mgmasl's concealment question, I believe that the rules are describing a special case for rooftops. That concealment is lost
immediately if a GO enemy unit (within 16 hexes) gains LOS to it from the same or higher level:
B23.82 CONCEALMENT: A Rooftop Location is Concealment Terrain only for setup purposes. A unit can remain hidden or concealed on a rooftop only if there are no Good Order enemy ground units within 16 hexes of it that are at the same or higher level with a LOS to it; these same restrictions apply to gaining "?" on a rooftop [EXC: the enemy units need only be unbroken (as opposed to Good Order) to prohibit such gain].
In short, rooftops are depicted as
especially visible and vulnerable to an enemy that is at an equal or higher level.