Precisely. Why was the note left where it was; or, to ask it another way, why did the Ramseys say the note was left where it was?
Thinking out loud: you sneak into the child's bedroom and snatch her; that's as simple as snapping your fingers, which for me isn't so simple; I'm all thumbs.
You're there to take possession of her and whisk her away to a safe hiding place, where she'll dine on quince and slices of mince (or is it the other way 'round?) and an occasional jelly bean as a reward for good behavior.
You've already written the ransom note and carefully worked out where you'll put it on conspicuous display. I guess you could write two identical notes since there are two sets of stairs leading from the parents' bedroom, but, no, you've settled on one. Now, usually when you snatch a baby from his carriage, you leave the note in the carriage; why, I can only imagine. So, why wouldn't you leave the note on the bed if you snatched the girl from her bedroom? Well, cuz maybe you didn't snatch her from her bedroom. But, you didn't snatch her from a lower rung of the spiral stairs either. So, where did you snatch her from? A diller, a dollar, a ten-o'clock scholar.
OK, this is a house we're dealing with, not a sunny nook in a city park. Maybe we (you, they) can leave the note at the door where we exit. Too risky; too many doors; it'll take the parents a fortnight to find the note. We've got reservations on a flight to Mexico City tomorrow night. We gotta put it where it'll be found "immediately". We'll pin it to the body; that way when they find the body they'll find the note. No, wait. That won't work. Let's see....we've addressed the note to John. John is a pilot. John likes to do aerobatics. It's obvious he'd prefer the spiral stairs. Voila!
Now, please don't vituperate me....