I am now 100% convinced that Barba’s secret is going to be something related to the public record regarding Lewis coming back to bite him.
My evidence is mostly circumstantial, but consists of:
1. The foreshadowing earlier this season- “in my world, that (lying to cover someone else) gets you disbarred.” “Yeah, well, in my world, it gets you a promotion.”
2. The fact that the secret is something he can’t tell Benson about. Let’s be real, guys- Barson are best friends. The only thing he absolutely could not tell her would be if he had raped or killed someone himself… which just would not happen, ever. Or… he wouldn’t tell her if he was in trouble now because of how he protected her in the past. Because he still wants to protect her, and doesn’t want her to feel that guilt.
3. The filming locations listed while this episode was being made include Forlini’s, which is the Barson “sidebar” restaurant.
4. With Barba getting into territory that borders on malicious prosecution this season (I.E. in Imposter), it could all add up to a huge disciplinary investigation. That kind of thing is taken seriously. They could even work in a ripped from the headlines story regarding Mike Nifong of the Duke Lacrosse case- Nifong was disbarred for his misconduct. They could have something in-universe where Barba is treated the same once the Lewis-related details come out.
5. Let’s be real, guys. Barba is not going to be a criminal. Further, he is a good man What is risking his career has to be a case of professional misconduct that is still morally righteous. Nothing else would fit for Barba. This would fit the bill perfectly.
This is the only thing that makes sense.