i have had false alarms before. I would check the sensitivity of the machine.. It is is to high it might give out a false reading..

If it happens again tonight turn the light on first and check her colour of her face.. If it is all normal then it is a false reading and just the machine.. so best way is to test it during the day

have you got a ceiling fan in the room? if so turn it on and adjust the sensitivity then turn the fan off and it should alarm.. hope that makes sense