Normally, when someone saw their teacher, even if they were not overjoyed, they would at least not feel anything negative. But Anko and Orochimaru were a somewhat different pair of teacher and student.
In Anko's heart, she felt she was the one who had been abandoned.
At first, Anko had actually been Orochimaru's test subject, his guinea pig. Orochimaru had experimented on her with the Cursed Seal, something like a power-amplifying rune. But its success rate was very low, so Anko really had been lucky to survive.
But did Orochimaru truly have no feelings for Anko? Of course not. If that were really the case, Orochimaru would not have taught Anko ninjutsu, taken her on missions, or merely sealed her memories relating to him when he defected from the village instead of killing her.
Actually, we seem to view feelings in overly absolute terms, believing that only a willingness to give everything for someone, or to abandon what one wants to do for them, counts as genuine affection. But feelings like that, whether familial love, friendship, romantic love, or the bond between teacher and student, are far, far too rare.
So Orochimaru definitely had feelings for Anko. They simply were not that deep, not to the point of abandoning his plans for her and staying in Konoha forever.
In fact, on the eve of his departure, Orochimaru had asked Anko whether she wanted to leave with him. But perhaps out of fear of Orochimaru, perhaps out of uncertainty about the road ahead, perhaps because she did not want to leave Konoha, or for some other reason, she had ultimately chosen to stay.
But that memory was still sealed, and Anko had yet to remember it.
So all these years, she had believed she had been abandoned.
In any case, so