It appears that you have one of the newest, self-testing GFCI receptacles installed, they perform self-testing cycle every fifteen minutes. That beeping sound indicates that that ground fault circuit interrupter protection might have been compromised. There might be visual indicator present on your GFCI device as well.
What’s puzzling is the timing of this event… I’m wondering if you have some type of equipment installed on the same circuit or plugged into that receptacle. Something that would be activated during the night and cause GFCI to trip, water softener system could possibly do that since it is usually set to operate during the night.
Please let me know once you determine the cause.