Sakura is a medic, she noticed thing out of the ordinary.Just to remind that they are not sensor ninja and they are vs super long range + quite powerful summon and if danzo has to go all out he use KA in the begining and end the battle.
Karin able to detect his secret BECAUSE she was sitting there and hide and had TIME.
Sakura analyzes her opponent in battle, she will notice everything he does.
KA isn't working, how will he control her? Anything drastic and her will power kicks in. He isn't binding her with it or anything.
This may sound like fangirling but no, Sakura really has too much going for her to be caught in such of trivial jutsu. Also it isn't a battle jutsu.