시나브로, sinabeuro: little by little, without your noticing.
Snow does it. Children do it. So does getting fond of somebody.
The not-noticing is inside the word. A thing can only be said to happen sinabeuro if you were not watching it happen — if you look up and it is further along than it was the last time you checked.
The snow this morning was over the top of the step, and nobody in the house had heard a thing.