I chose to believe that Duke Thomas, with the utmost care, likes to hide liiiittle tiny bat-themed objects around the manor. Like a hidden mickey situation. Tiny bat symbol at the corner of a painting, taxidermy bat mixed in with a shelf of other Knick knacks, a statuette on the fireplace that looks just a little bit like the cowl at certain angles. Because hey, it’s Funny.
It takes Bruce a while to notice and he’s naturally pissed (that it took him so long, but also) that this could blow their entire operation.
Which means, naturally, that Duke will not stop, but will instead have to get more creative.
Cut to a month after Bruce has told Duke in no uncertain terms to “knock it off” where he gets cozy in bed, about to sleep for the first time in three days, when he realizes that Duke has replaced his entire bedframe. It’s an almost exact replica, almost, but carved into the intricate design, small enough to miss, is a bat symbol tucked amongst the leaves and foliage and what have you.
Cut to Bruce ransacking the manor, uncovering little bat symbols all over the damn place. Under rugs, behind paintings, imperceptible unless you are really looking.
Cut once more to Duke, half giggling half screaming, being chased from rooftop to rooftop by a very angry Batman.