Is this a Libre2?
Using phone or the reader?
Do you have the 'Signal Loss' Alarm option on?
I always keep that off, because it does occasionally briefly lose connection, and I wouldn't want it beeping every time.
What was the reading when you checked?
Another possibility is that it was heading low and predicted* a hypo, but the adrenaline from being woken by the alarm was enough to nudge it back up so it never crossed the hypo threshold.
* the readings it gives try to counteract the ~15m delay from reading interstitial fluid by extrapolating the recent rate of change forwards. So the reading you see and which triggers alarms contains an element of prediction, and when you look back at the graph it might never have actually reached that level.