Smoke Detectors can go off when they detect a slight amount of smoke such as from cigarette smoking and even cooking smoke.
If you're cooking something on the stove and there's some smoke or if you're baking and the oven is not clean and is producing some smoke then the smoke detector can go off.
Another reason smoke detectors go off for no apparent reason is because the battery is low or going dead and it needs replaced.
The smoke detector will usually chirp every few seconds or so when your battery in the smoke detector needs replaced.
Smoke detectors can also go bad as well and if it continues going off and there's no smoke and the battery is good then the smoke detector could be bad.
A smoke detector should be replaced every 5 to 10 years and if your smoke detector is 10 years old or older it should be replaced.
The smoke detector may also be dirty and need cleaned of dust as the dust buildup could cause the smoke detector to go off as well.