The threat timer shows the period when your base can be raided. However, the chance that your base will be attacked and items will be stolen or, at the very least, the walls will be broken, is not 100%. This can happen if there was an unsuccessful attempt to raid your home (the raiders did not break the walls and did not take anything). In this case, the timer will disappear regardless of the result, since there was still an attempt to attack the base.