After Kenny's death (which ever one you see) Christa and Omid seem to be wary of Lee. With the "they will remember that" comments and their looks to each other, kind of lead me to believe that when Lee crosses the sign and are separated from him, they wait just long enough for Lee to be out of site, then ditch him.
I mean, here's a guy that decides to go on a suicide mission through a herd of walkers....they had to think at that moment that he's not going to make it. I think Christa would talk Omid into getting out of Savannah while Lee made a distraction. She is pregnant with Omid's child, they have to think about themselves, they've barely even been with the group but a few days...how much loyalty to the "group" do they really have?
To me this would explain why no matter where you tell them to meet you...that they aren't there.
I don't know if the two people on the hill at the very end are them or not, to be hopeful, I would like it to be them.
But I could also see season 2 starting out with Clementine having to run from "strangers" who are dangerous rather than finding Christa and Omid.
Just my thoughts.