Usage doesn't seem right

[#1]

I'm sure this question has been asked before, but the figures that PlusNet are showing for my usage cannot be correct,
Firstly, we are are very security aware and have a very strong password on our wifi, which is changed regularly.
There are the following discrepancies on our usage:
Overnight usage of up to 18MB per night on a regular basis - we are always in bed by midnight and don't use the internet before 8am, so how is this possible?
Usage of 45MB while we were away for the weekend?!!
It makes me wonder what other inaccuracies there are...
Any ideas anyone?
Thanks
PS It is soooooooooo annoying that PlusNet don't display an email address.  UK legislation states that if you are selling online then an address and email address must be easily found.  No chance.

[#2]

Is your router on during this time?
Sounds like typical internet noise which everyone gets - these are the spammers/scammers/automated bots that are trying to scan/infest your network and PCs all the time. These are normally blocked by your routers firewall but they do take bandwidth between plusnet and your router so count towards your allowance. Nothing you can do about it except turn off your router... but this could be seen as a faulty line and cause a reduction in line speed so it;s swings and roundabouts on what you can do.

[#3]

Mmmm.  Surely that's happening in the daytime also then, which means that approx 2GB of "our" usage isn't being used by us at all.  I can understand 1MB or so, but 18MB in a night? 

[#4]

Yes that can register as usage up to about 18/20mb in my experience. I would not expect this to be a regular problem for you though.

[#5]

Does the Plusnet firewall stop this background usage being counted against the account?
https://portal.plus.net/my.html?action=firewall