Did you set up the sense for notification on Alexa or Google or some other 3rd party app? Some people have set alarms on Alexa and forgotten.
https://help.fitbit.com/manuals/manual_sense_en_US.pdf#page34
if your having trouble accessing the alarms on sense:
On sense swipe left til you see alarms and tap
alarms and you should see a list. If you tap on the time of the alarm you’ll see on and off. Tap off.
To remove it swipe up til you see remove and tap it.
It will ask to confirm. Do that and it’s gone.
https://help.fitbit.com/manuals/manual_sense_en_US.pdf#page42