I don't want to speculate what Data smells like. I always found what he looks like revolting enough (I really do not like Data).
From the cat's perspective, the instinct is to mark one's human as a way of indicating "this is mine" to other cats. So Spot would have rubbed his head and muzzle on Data's face and tried to knead his skin (or at least knead his lap or arms through his uniform). I'm guessing that Data might have allowed this from having read that cats do it, but the emotional connection would have completely escaped him.