Free usage problem

[#1]

Not sure if this is the place for this ..... move it if necessary.....
I have a D-Link DSL 2640B Wireless Router.....
My laptop, and my wife`s laptop both have access to it via wireless connection.
I also allow "visitors" (family/friends) to access it if they need to....
The problem is, I have noticed a couple of nights a week,  about 4mb of usage on the "free usage" period after midnight....
in particular today Saturday 25 April 2009, it shows that after midnight 4.3mb of usage was recorded..... however, I know that my laptop, and my wife`s laptop were both switched off before midnight, and not used until well after 8 a.m.
Ok.... so it is "free".... the normal overnight usage is about 312kb (presumably just kicking in every now and again, to keep the line open) there could be a problem, IF this "unauthorised" usage starts to appear in my "normal" daily usage chart.... I will not be able to spot it, and it could take my usage "over the top".....
None of the "friends and family" are likely to be sitting outside my house after midnight, so it appears someone, somewhere is tapping in to my router....
can anyone advise me how (apart from disconnecting from the mains) I can prevent this...
On the set up, I changed the password from admin to X X X X X X X X  as advised
I gave the router a name, and I also applied a  WPA psk  key
Looking at the settings on the router, it shows my laptop, and my wife`s laptop, and an "unknown"  (which could be my granddaughters)
If I block the "unknown", and find it is her laptop..... can I go back to the settings and "unblock" it

[#2]

312k overnight? mine's usually about 27k ... strange

[#3]

Can you set up an access list on your router.
If so, just allow the two you know and when the granddaughter comes round see if it is hers. If it is just add it to the list

[#4]

sometimes it takes a while for the counter to update.  it could be registering usage from when you, or you wife were online.

[#5]

@artificer....
The usual "delay" is about 2hours..... e.g. if I download a film at 15:00, the usage will not show up until at least 17:00
The usage I have mentioned occurred between midnight lastnight (00:01 SATURDAY) and 08:00 this morning ( same Saturday)... it is now well over 12 hours since the free period closed, therefore the usage counter would have been updated by 11:00 at the latest.... and I noticed the reading at 19:30 hours tonight.....

[#6]

Update........
I was advised to "hide network" in the settings, and lo and behold, my "free usage" last night was just 12.74 kb......
hmmmmm......Interesting............. verrry interesting ! ! !

[#7]

If someone had hack your WiFi hiding the ssid will not stop them as the hacker would already have the details. 

[#8]

Here is a copy of my months FREE usage...
SUN MON TUE WED THUR FRI   SAT
                      365KB     476KB 4.7MB 3.7MB
351KB 353KB 1.1MB 3.2MB 12MB 352KB 347KB
365KB 342KB 343KB 544KB 365KB 3.4MB 8.5MB
365KB 350KB 370KB 365KB 4.7MB 425KB 4.3MB
12.7KB

Thursday midnite to Fri 0800, and Friday midnite to 0800 .. ( Friday and Saturday figures) ... certainly look suspicious to me ! !...