Fridge Logic: Why didn't Darth Vader sense Han in the closet when he strangled Greelanx? Or assuming he did, why let him go?
Wild Mass Guess: He probably did sense him, and let Han go to bear witness to what happened to traitors. After all, if he didn't have someone listening there was little reason to bother with the charade of acting like Greelanx had betrayed the Empire, when in fact he'd been ordered to lose (the Hutt bribe was just unrelated icing on the cake).
"Funny Aneurysm" Moment: Han makes many smuggling friends, especially in The Hutt Gambit. Of all these friends only Chewie, Roa, Shug, and Salla don't end up betraying him later.
Nightmare Fuel: Han overhearing Greelanx getting Force-choked by Vader for accepting a bribe and throwing the battle of Nar Shaddaa.
Bria's farewell letter to Han in the first book, and his reaction to it.
Jarik's deathbed confession to Han that he lied to him about his name and was never related to him at all. Han says he always knew, though it doesn't matter and he's more than earned the name, and makes damn sure his death certificate reads Jarik Solo.