The data from 19:55 on 20.02 until 7:55 on 21.02 is only the night 20/21 February, and adds up to 28.6 MB, so pretty much between 20-30 MB per night.
That is mobile data use when I did not use the iPhone, while it was in the original Apple charging station, connected to an original Apple power plug, showing that it is being charged, and connected to a WiFi network setup by an Airport Extreme base station. I actually tried a second (also Apple) charger, without the docking station, that also charged the iPhone, but the mobile data usage remained.
I do not see why my mobile phone should use 28.6 MB of mobile data per night while it is actually connected to the WiFi network.
@ diesel vdub: If Wi-Fi is turned on and connected, Cellular Data is on, and the device is connected to external power, mobile data usage DOES NOT stop, but instead is using the mobile data as described above. That is the problem. During the day, when the iPhone is not connected to a WiFi net, this usage does not occur (see usage 19:55 and 7:55, before and after it connected to WiFi network).
@ diesel vdub: I have manually killed all apps before putting it in the docking station. If it's a rogue app, it must be one that I cannot kill. Even if it is a rogue up, should it not use the WiFi connection?
@ extremecarbon: Current usage would add up to more than 500 MB per month for actually no usage by myself - the usage during the day when I actually use the phone and its internet connection is on top. I am not aware of other smartphones that do that, nor did my 3G.
Thanks for your support!