" I really need to see the minute-by-minute usage records to be able to rule that out.":
There is no minute by minute but there is the traditional Hourly History Report.
Go here, select your modem type, enter your SAN .. remember to capitalize the letters DSS, and select a date:
The only issues with this display are :
It is written from the perspective of Eastern Time Zone so if you are in another corrections must be made.
It is a HISTORY REPORT and as such is not "real time" but has a usual 1 to 3 hour delay.
These are "gross" usage numbers and not all indicated usage is actually charged against your allowance.
That report looks like this:
Additionally, others have suggested seeing the Download Status Meter utility not showing usage drain .. and then suddenly "catch up" with a big drop all at once.
Of the three methods to track you usage (SCC, Download Status Meter, "At A Glance" Meter), the At A Glance Meter is the most accurate and dependable.
It can be found here, after logging in:
Usage can be tracked at the MB level:
Just to show how "slippery" things can be I have noticed in the last two evenings that my ratio of upload to download is out of wack.
Now it just HAS to be an "authorized" device accessing my wireless network.
It can only be one of two possible devices, an HP wireless printer that is set not to access the web or my Nook HD+ that, as much as is possible with those types of devices is set not to "auto-date" or "sync".
Yet something on my network uploaded 2.2MB between midnight and 1am eastern.
All other devices were "hard off".
I know that is a far cry from 2.5GB, but the principal is the same ... unidentified and unexpected network activity.