Great question — answering as the UphillTech team:
Poorly filled screw head hole (or nail-head hole if you have drywallers using 1970's and earlier methods - likely a screw head) It almost certainly has been there the whole time, you just haven't noticed it until now. It was built and painted like that, and now you are looking at the ceiling for defects, so you found it. Very unlikely that it "appeared" recently. Usually a poor job due to rushing - moving on before the joint compound has fully dried, and it shrinks further making a dimple. Could also be plain sloppy work not even filling it properly in the first place, but that's less common. In any case, fill it, let it dry completely, check it very carefully, and only then repaint. if you...
Full answer by Ecnerwal: https://diy.stackexchange.com/a/331759 (CC BY-SA 4.0).
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