Although it's been established Ava was manipulating Caleb to escape, the pair's final interaction throws in some emotional ambiguity. After assembling her new exterior, Ava completely ignores Caleb as she heads for the outside world. However, just as the doors close, she takes once last look at him.
It's a purposeful glance and one that undercuts the murder and general cold demeanour of the previous ten minutes. Of course, it could be just a human tic, but it's at such an important moment it's hard to not read more into it.
Even though she was merely using him, Ava's inadvertently developed feelings for Caleb on account of his doting - a reversal of how Caleb himself originally began to like Ava. It's slight, but that moment acts as further proof of Ava's humanity, her personality developing emotional contradictions.
What Happens To Caleb?
No matter how Ava feels, she still leaves Caleb locked in the facility, just as he intended to leave Nathan. Both the A.I. creator and his mute assistant are dead/destroyed respectively, but the young coder is very much alive - what's going to happen to him?
The simple answer is that he'll be left there until he either can break out or dies, and given how secure the facility has been set up to be, the former seems unlikely. It's pretty morbid and highlights just how dark Caleb was becoming under Ava's influence - he was going to subject that fate on Nathan. Although Ava obviously couldn't set him free (he'd blow the whole thing), there's the possibility she's purposely left him in this state as retribution for what he planned to do to his boss.
However, an ambiguous final scene suggests a potentially different fate. The last time Caleb is seen, the power shuts down. His late-night coding reworked the lockdown procedure so that a power cut now unlocks all the doors, meaning that as she left Ava may have given him an opportunity at escape, a further show of a potentially deeper emotional connection.