I have a mad conjecture that Odium visited Scadriel during Hero of Ages. Remember when Vin was jumping around Yomin's city, gathering information from various sources? She landed near a possible source, but her instincts told her to flee, so she did without question. Now, it's possible that Brandon was just using this to reinforce Vin's character. But, as a Mistborn, there are very few things in that city that could threaten her, and I think she'd been a Mistborn for long enough at that point for her instincts to have adapted to her power. She could feel that something dangerous was in that alley.
What would Odium have seen? Leras dead, Preservation's power broken, Ruin's power bound, all on a world mere weeks from destruction. He'd not have needed to help it along—indeed, why bother?
Now, if indeed he ever does learn of Sazed's ascension, there may be trouble, but I wouldn't count the Terrisman out. He's got two complimentary Shards where Rayse has one, and he's frickin' Sazed.