If Echo wasn't wearing the helmet, his expression would have read clearer seconds after Jon recognized who he was. Even if it is a generalized 'clone trooper' title – that is what he is.

Perhaps he thought the chances of running into anyone from the galaxy were slim to none at this point.

"Yeah." It's an faltered reply at first, but he recomposes himself, taking a moment to study the Archivist. "You seem to be familiar with us."