The "phantom usage" could also be just lousy bookkeeping on the carrier's part. A lot of them trigger billing events on a "per session" basis, and if the connection is kept alive (as most smartphones do), then the billing isn't cut off till the middle of night, usually midnight in the time zone wherever that carrier has centralized their billing.
Since the cutoff is the middle of the night, the billing system "charges" all the usage to what looks like a middle-of-the-night session. So, you see usage in the wee hours when it was actually legit usage during the day.
If the usage is pushing against your plan limit, the best thing to do is go into your cellular usage and see if any apps are using excessive data.