Just bought house with hairline diagonal crack from window
Probably just the house settling - could be due to:


Underlying soil conditions causing the house to rise or fall
Changes in seasonal humidity causing the window casing to shrink
Perhaps you didn't notice it until it got dark and cold so you're inside more
Moving in: if the house was empty, then filled with all your belongings, it can actually cause the house to settle.  Happened to my parents' house.


Probably not a serious structural issue (IANAL). If you patch it, make sure to use some reinforcing mesh to keep it from just cracking again.

Perhaps this question might also help.