First up, I understand these kind of things are best left to experts' inspection and internet forums won't help much. I have called an engineer but until he arrives in a day or two, for my solace, I am asking this question. FYI I am just short of having a panic attack!
About the house
Ours is a three storeyed house about 35 years old. It is a brick wall and concrete plastered house all the way, typical of India, and has 11 columns in all. Till 2011 there was a small room on the third floor and the rest was terrace. In 2011 the room was extended incorporating the entire terrace. Before the extension we had contacted an engineer and he inspected the house and gave a go ahead. The problem is in this new floor.
About the cracks
Lately, possibly from the beginning of this year, I have noticed hairline cracks on all the walls. Most of them are thin and vertical. They don't go all the way. Some start at the top near the ceiling and stop midway. Some start midway and stop midway. None go all the way to the bottom to the floor, and none rise from there. Only two cracks have a vaguely diagonal shape but they don't continue all the way and stop midway. All the cracks stop abruptly. Although the cracks don't seem to be mirroring on both sides of the wall, they are present in some places (not all) and close to mirroring.
The cracks don't seem to be widening but I get to see more cracks the more I look. I am not sure if they were always there or are they developing newly. I am very scared TBH. The reason I am panicking is I have absolutely no clue about the seriousness. My father thinks they are just air cracks on the plaster. We had called the contractor (not the engineer) a couple of months back and he said the same.
What is a good layman's way of determining the seriousness of the cracks?
I have two related questions:
Isn't it a bit late for the structural defects, if any, to start showing up after three years? Shouldn't it be apparent during the construction or within the first few months?
If it is indeed structural, against my hope, will the house collapse without warning or will there be telltale signs?