Ok so my crush just whistled when I walked past why can't he just say Hi? Please read the rest?

It was a one syllable whistle that even I do when something was close to falling or something that was a close call or dangerous as in driving or something. What did he mean by the whistle and why didn't he just say Hi?
Ok so my crush just whistled when I walked past why can't he just say Hi? Please read the rest?
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