Spike can feel Brock's look, which seems more effective than checking while he's still in the process of shrugging off his agitation; he just assumes that it's still all about the outfit. Tempted as he is to leave Lucrezia with the two of them, refusing to do what he came to do would make it even more obvious.

"That must be it." -- There's a possibility he only interjects because he can tell when he's being ignored. Then he takes a step back towards the garage, which is where they should have stayed to start with.