From the dialogue of their last scene together, Deb said a lot of stuff to Dex about learning that people weren't who you always thought they were, and about secrets and deception. And while all of this could refer to her earlier revelations about Harry, and her recent squabbles with both LaGuerta and Quinn, it could also be interpreted as referring to a strong suspicion of Dexter and his true nature. To tell ya the truth; I not only think that Deb knows the truth about Dexter, but that she's known in her heart of hearts for a long, long time. Perhaps even since childhood. In fact, I'm beginning to suspect that her becoming a cop had more to do with her wanting to help protect Dexter than any desire to follow in Harry's footsteps.
And as for Lumen leaving... Well, we've seen her get as far as the security check at the airport, with boarding pass in hand, and it came to exactly nothing. Moreover, Julia Styles let some info slip earlier in the season about the relationship between Lumen and Dex and where it was ultimately going, and the relationship she hinted at didn't really materialize. The question is, did it not materialize because TPTB altered their plans; was she simply wrong or projecting her own wishful thinking; or have they simply not materialized yet
, because the relationship still isn't over? I'm guessing the latter, and think we have not seen the last of Lumen. Not by a long-shot.