Really, there generally are 4 possibilities.
1) Something run amuk localally and downloaded it all. Possible? Yes.. but for that much, a little iffy...
(Seen stuff run amok and get stuck in a downlaod loop, download say windows updates for 3 days straight, but not THAT much usage)
2) Modem cloneing. I dont think this happens that often.. but still a possibility. Maybe only one day if they figured how to kick the other one off?
3) Someone broke into your network and did it. Even with the most secure password.. there is always still the possibility. There is a vulnerability that has been around lately, with a good chunk of 3rd party wireless routers, where someone can get access fairly simply through WPS... there are tutorials on how to do it online, so joe blow nextdoor could possibly easily do it.
4) A rogers counter error. IF it was a general error, counting too much.. i would think it would do it more consistantly, and not on one day only, etc.
Some people say that rogers specifically adds usage on, just to screw us over and make us pay. Personally i dont beleive it.. as why . someone off, so they would leave, and then loose the overall revenue over a small amount income from the overage?
Generally, i find mine to be accurate... only times there are odities.. i can usually explain it.
(Oh yeah, downloaded that new game so there is 15g.... or like yesterday.. forgot and left a torrent on for like 5 hours.. had like 4g of upload usage... whooops)