He notices the stiffening, and the sudden crispness, and his expression changes to concern as he quickly runs over options in his mind for politely taking leave of the girl before she starts denouncing him as a murdering madman and wakes the sparklet up. His reaction is assumption, but he's had people respond that way so often, and it's so hard to stop Optimus from crying once she's started that he might not be able to help it at this point.