This does happen from time to time. I contacted support about a outlet that turned on a year or so ago at 2:00am. They basically said, that happens every now and then. Other devices over my time with ST have done this as well.
I used the outlet to turn on a space heater in the bathroom, so I was a bit concerned about it coming on randomly. I had written a smartapp to monitor it and since it came on during a time it was not supposed to the app turned it back off. Support reviewed it and said they showed it being turned on manually, but it had not been.
If it happens on a regular basis then it is probably as suggested above some automation setting it off you forgot about or set wrong.
[edit]After posting this I went back and looked because I thought I had posted about it when it happened. Here is the thread.