Hi support-jo, welcome to the forum!
It does not appear that you've been monitoring this device for very long, only a few hours. Might it have supplied a false positive in one poll? I do not believe it's likely it would be a transmission error in the serial line, but it could be. I am certain that Mango only processed what it was given, whether that was the serial bus or controller (or maybe someone imported an artificial value)?
You could increase the duration of the state detector for Fire_Found to last a few polls if the sensor reports brief false positives. Or figure out why the sensor would have alleged there to have been fire.