Smirk
Jim doesn't know Batman's identity. He can truthfully swear he has no evidence whatsoever on the matter.
If he is honest with himself – and, really, about this, he can't be with anyone else – he doesn't want to know. And, sometimes, it becomes quite difficult to keep it that way.
The man himself gives nothing away. At first, Jim thought prisons had more smiling doors. It took time for Batman to relax enough to let go a bit. Nowadays, Jim considers him a friend, though a secretive one.
But the kids…
It had been hard enough to ignore the familiarity of the first Robin's mobile face. It's not like he regularly spent his Saturday evening with Babs in one way or another.
At some point, Robin disappeared. Coincidentally, Dick Grayson moved out of Gotham.
Right.
But still, it was just a gut feeling, nothing concrete, nothing certain.
Then a new Robin appeared. Jim will always remember that night when Batman dropped with him on the roof where Jim was smoking.
"Jim."
"Hi, big guy."
He is able to call him Batman with a straight face in public but, seriously. No way he'll do the same in private.
"Hello, commish' Gordon", the new kid greeted him.
Everything in his voice and accent sounded like Gotham's streets. For a split of second, Jim didn't know why – then the kid smirked and he almost swallowed his cigarette. He would have recognized that smirk anywhere.
"Something wrong?"
Concern was barely audible in Batman's voice, but it was there nonetheless. Jim could only clear his throat, nod, and explain their latest lost case against the Maroni's.