robin is a flirt, she flirted with jinbe in the past ('i can't say no to a handsome man ;)'), and she's flirting with law now: 'oh you're here? you must have liked my perfume to follow me all the way here ;)'
in truth, while it's not impossible that law likes robin (as a woman), it's even more likely that the actual reason he's stalking her is that he wants to kidnap/seduce her so that she'd become HIS poneglyph-reader. or that he's just following her around to get bits and pieces of the ancient history.