unexplained usage

[#1]

The household went away over the weekend and all devices were logged out and turned off. However, on the days when nothing should have any access to broadband the usage on plusnet appears to be
24 Thu 25 Fri 26 Sat 27 Sun
Allowance Used 57.24KB 53.09KB 54.07KB 53.09KB
Overnight Usage 27.13KB 33.98KB 26.58KB 24.98KB
Total 84.37KB 87.07KB 80.64KB 78.07KB
The router has no devices plugged in directly and wifi is WEP2 passworded and also the wifi is restricted to named MAC numbers.
Is there any innocent explanation for the usage? It isn't particularly big and the night time usage appears to be in the region of half of the total usage.
Any thoughts out there?

[#2]

There is, I think, a small amount of data passing back and fro all the time even when you are not using it.
Along the lines of "keeping the connection alive" sort of thing - think of it as one end just asking every now and then to the far end "hello - are you still there?"
The other end then supplies a few kb of info to prove it still "there".

[#3]

Hi robalb,
It's pretty much as x47c says. Here's our explanation.
Jojo

[#4]

Thanks for the answers.
I am disappointed though, I was getting quite excited at the thought that the house might have a high tech ghost, a body carrying its head and smart phone under its arm. There must be such things out there - it cannot be just zombies, vampires and werewolves.

[#5]

I presume you mean with a 'heads-free handset'

[#6]

That's one of the best I've seen on here for a long time!