Sylvia guessed that buying soap wasn't too unusual. How else were they supposed to bathe if they didn't have soap? She followed Pylia to the pier, and waited, keeping her eyes on her. It seemed like Pylia was goofing off at this point. But it still didn't explain her behavior since that night...
As she waited and kept her eyes on Pylia, she wondered if mind control was possible. After all, magic did exist here, who's to say that magic can't be used to affect minds. 

She wondered if Pylia was at the beach merely because she wasn't thinking and was just calming herself down, or if she was waiting for someone or something...