What detail in the beginning of "There Will Come Soft Rains" foreshadows the eventual realization?
Like many science-fiction stories of the day, "There Will Come Soft Rains" opens with a futuristic marvel: an automated house which speaks to wake its inhabitants up, gives the date and a to-do list, and makes breakfast. It is strange that no human is mentioned, but the first hint that something is truly wrong comes when the house tells the time again:
...no doors slammed, no carpets took the soft tread of rubber heels. It was raining outside. The weather box on the front door sang quietly: "Rain, rain, go away; umbrellas, raincoats for today..." And the rain tapped on the empty house, echoing.
(Bradbury, "There Will Come Soft Rains," nexuslearning.net)
This moment shows that there is nobody living in the house to take advantage of its preparations. The house cannot detect life itself, and so can't understand that the humans are gone. The full impact of this knowledge is not shown until the images of pain burned into the wall by nuclear fire are shown, but these are the lines that begin the story's gentle decline into the madness of a house created by humans, for human use, without any humans to use it.