Mysterious Usage

[#1]

I checked the VMBU this morning to find it said 708Mb.  Then again about 15 minutes ago approx 13:30)  and it said 1.98GB.  I've just checked it now (about 13:45)  and it says 2.03GB.
What's odd about this is that there's nobody home and hasn't been since about 8:30 this morning.  Everyone's either at school or work, and  I'm connecting via VNC. I've checked my router's logs, and there are no mac addresses that have attached that I don't recognise.
However...one of the attached devices is the Sky+ box.  Is there any possibility that this could be the culprit?  
-Dave

[#2]

Did anyone start downloading something before leaving this morning? The latest movies are 4 gig+ these days in HD format.
Sky On Demand shouldn't download anything on its own - the showcase stuff is recorded overnight OTA.

[#3]

Cheers Andy
Nope, nobody should have done anything.  I was finishing off a 10GB Steam download at about 6:30am after which I disconnected the laptop.  But that data shows up in the Overnights.
My son might have been using the interwebs before he got up, but that would have been no later than 7 in the morning, which would have put it before the 8am Peak start.  I checked when I got into work (at approx 8:30am) and it already said 703Mb, which I put down to one of the kids watching Youtube before school.
I did copy three 50Mb of MP3 files across the interwebs to my PC here at work round about 12:40 but that's it.  Certainly not 2Gb!
All the indications are that no wireless intruders have been on (the router emails me logs regularly, there's no unusual DHCP requests in there).  just that Sky Box.
And now I look back on my previous records, it looks like there's never less than 1.2GB of peak usage every day over the last month.
EDIT:  Although we've got the Sky box plugged into the LAN we've never actually used any of the Anytime/On Demand features yet.  I just plugged it in a few months ago to see if it picked up an IP and I've never bothered to unplug it since.  I might disconnect it when I get home today.

[#4]

There's no change one of the kids could have been watching videos from a channel on Youtube and left it running?

[#5]

Nope, because we only have two desktop PCs (the ones I VNC into) and neither of them had a browser open when I logged into them.

[#6]

Looking at my TB graph, there seems to be a lot of activity between 9am and 12pm today, just before I lost access for about 10 minutes when my router disconnected. Now that wasn't me.  i was connected in, but I wasn't doing any more than browsing.
jim:green tag fixed mod:end

[#7]

VNC will use a fair amount of bandwidth, depending on resolution, quality, and how much the image changes.

[#8]

VNC? Really?  Of the order of 2 gigabytes?

[#9]

Depends on your resolution and colour depth you are connecting at. Also the time it is connected.
E.g.
1280x1024 = 1.3Mb multiply that by the colour depth and you have a large number. Compression helps here, if both ends support it.

[#10]

Hmmm...1280x768 with a colour depth of 24.  I guess that's where my gigs are going.  How do I calculate that out?
EDIT: crikey, that's 2.8Mbps. 

[#11]

Well, it's 2.8 MB for one whole frame. VNC can use compression, and only send the parts of the screen that have changed, but also consider that only 1 frame per second would appear rather jerky. I think VNC can also adjust the level of compression and how frequently it updates the screen based on how much bandwidth is available, so if you have fast FTTC upload, it will use it to give nice quality.

[#12]

I have reduced the colour depth of my VNC connection and the data usage has dropped considerably. 
Thanks to everyone who helped me out here.
-Dave