Slowly the thought occurs that there's something familiar about that arse, and about the brolly he's wielding rather expertly in an attempt to dislodge the child's plaything stuck in the tree before his store... And when he fails to knock the little aeroplane free and the toy suddenly seems to just leap into his arms... That's when it dawns on her, he must be a wizard, and in that light - helped along by another glance at his arse in confirmation - comes the absolute certainty: not just any wizard at that.
Really, she should have known from the name of his establishment, or at least suspected. 'Serpentine Scrolls'? It didn't exactly scream 'Muggle', now did it?
No, in conjunction with that magnificent backside, it screamed 'Professor Snape'.
For erexen, my sister by another mister, with oodles and oodles of squishy hugs.