Look at Suyin's face just when the others tell her that Aiwei was a traitor. Her eyes didn't widen in shock -- they darted back and forth like someone who was trying to hide something, afraid of being found out. It wasn't "I can't believe my closest advisor would betray me," it was "Uh-oh, how much do they know?" She hid it very quickly, but her attitude afterward was very calm, sounding more like mild concern than genuine shock, betrayal, and anger.
Also, consider the inconsistency. Suyin's city is all about ultra-tight security. They even encase the entire thing in huge metal domes every night. This is not a woman who takes chances with the safety of the people under her charge; if anything, she errs on the side of too much protection. Yet she encourages Team Avatar to sneak out on their own at night, without anyone knowing where they're going, in pursuit of a gang who we know to be after Korra and powerful enough that Team Avatar has no chance of defeating them alone. There's no way someone so overprotective would do that with anyone she genuinely wanted to protect.
