Mysterious hijacked data download

ever since i got my iphone, been working great.

but had 2 identical incidents in the past month that is really strange.

i live in canada (mobile plans suck up here), and i'm limited to 500 MB per month. my normal habit is about 200-300 MB per month (use wifi, which is unlimited data).

i had 2 incidents where i left the iphone in airplane mode (i set it to airplane to let my daughter use the phone for games) overnight. in 2 incidents, the phone magically downloaded 3GB and 400 MB overnight! and these happened in the middle of the night (the 3 GB download happened at 3 am).

i called service provider (rogers) and they did cancel the extra charges for the 3 GB download, and thought someone hijacked the phone to download.

has anyone heard of this happening? i'm pretty sure it has something to do with the airplane mode, as this was the only commonality between the 2 incidents.