Post spoilered just for the hell of it.
Spoiler!
I dunno. The way Louis figured it out was pretty silly. For all we know, Mike could have kept asking about the key simply to have Louis reminisce on how he became Magna Cum Laude, not because he didn't know what it was. (Mind you, now that I'm typing this, I think Mike may have repeatedly mentioned that he didn't know what the key was for... so I could be simply writing all this for nothing
)
That said, seeing Louis get angry at Jessica like that was both a little scary and satisfying. Scary because it's a side of Louis we've never seen before, and satisfying because I can't stand Jessica and I'm glad to see her now in a position where she's fucked either way. Definitely sets the scene for what's very likely going to be a very icy and terse work environment. Though unlikely, I'd like to see Katrina back. She's very much an enabler, but it's nice to see someone who is on Louis' side, regardless of anything else. He definitely needs someone like that.
As an aside, it's nice to see Mike and Harvey finally be able to joke around with one another again.