Jack hesitates, waiting for Sinclair to make the first move, then gets up and shoulders open the door. It provides a nice distraction from actually having to answer.

What was he doing. Hahahaha.

He doesn't actually know; all he has is fake memories. And in his bitterness towards Fontaine, he doesn't want to repeat them to Sinclair, even if the alternative is not really having a response.

"I..." oh man making up a good story does not come naturally, why couldn't he have gotten that from Fontaine "...I was flying to... Britain." Yeah. He gets the door open and steps out onto the dock. "I had a job."