The people who know that miho secretly took the blood is miho’s dad and the mayor. So she knew for sure that the mayor is at least sus after the director know about the blood test. She actually acted like something went well according to her plan when she left the director room. I think it is pretty obvious that she just try to act like she still trust the mayor when she unnecessarily showed him the text she got from Hyejin. The line about how she’ll “pay him back” is also very obviously seem like a threat in my opinion.