The idea of Lena doesn't know Kara is Supergirl hence Kara's guilty face is plausible, but I wish they won't go that way because I think it's a disservice to her genius character not figuring out that her only two friends are to same person :/
I agree completely anon. I mean Lena is so incredibly smart and I truly do think she figured it out a long time ago. I think she is just waiting for Kara to come clean because she trusts her and probably trusts her reasoning for keeping it a secret.
However, the idea of Kara looking guilty because of a lack of revelation could still be a possibility due to the fact that Kara doesn’t know that Lena knows (if that is the case). So if something did go wrong with Jack, Lena could be understanding because she knows Kara. She knows that Kara would have done everything that she could have and would never do anything to intentionally hurt her. So once again, Lena could be sad but is also approaching the situation from a place of love, respect, and understanding toward Kara because she knows how hard making these types of decisions can be. Sometimes when you want to see justice done, people may get hurt in the process.
So whether Lena knows or not, Kara’s face would remain the same because Lena could also be keeping a secret to protect her and their relationship.