They're both summoned to see Snoke. Kylo's late. Destroying another console kinda slows you down a bit, you know how it is.

He's about to enter and he pauses, because Snoke is asking what Hux thinks he's doing conducting a relationship with Kylo. Kylo snickers and figures he's gonna let Hux field this one.

Only something's wrong because Hux just scoffs. Says that it's nothing serious, it's not a relationship, it's just letting of steam. He wouldn't want that with Kylo. He's too volatile and immature. etc etc.

And Kylo is just there like. Oh.

He had no idea.