Luckily, Jack's a masochist and therefore enjoys it too - as long as he gets what he's after in the end! And that move of Spike's can't fail to draw his eyes down to the newly shapely form of Spike's chest and, yes, he can tell there's no bra to get in the way of groping, if he's allowed to grope. He'd really rather like to. He misses getting to grope breasts.
He lifts his eyes back up to Spike's face and smirks. "Well, if you're not going to tell me, I guess I'll just have to do some exploring of my own!"