mysterious water in sink

I had an interesting puzzle this morning. My Duetto (2003) is still in winter mode, with all the water tanks drained and the pump off and taps left open. The same as I have done every year.
Every two weeks I put the van on charge. This morning when I put it on charge, I went inside to switch the main switch and vehicle switch on and noticed the washing up bowl was floating in about two inches of water in the sink. The plug was in the sink, which is how I leave it, to stop any smells coming up from the waste tank.
I know that the sink was empty two weeks ago, as we keep a moisture trap in the bowl and I changed it then and no water was present.
So where has the water come from in the sink??? There is no wet anywhere else that we could find and no leaks are evident in the roof. Plus any leak would have to be very precise, as there isn't much gap between the bowl and the edge of the sink.
We are stumped, any ideas? 

Well, we had a good look at the van this morning and there was a small film of water in the gap between the back of the cooker worktop and wall.
So our best guess is that one of the roof vents is leaking around a seal and the water is running down the inner wall and coming out by the cooker. No idea how the water is ending up in the sink though, but everything else is dry, so grateful for that.
Once we get a few dry days, I'm going to use clear silicone around the roof vent joints and hope that cures it.

Is the tap over the sink?

Wondering if even atmospheric pressure changes are moving it somehow, I assume the pump switch is off? If not the tap could be leaking with the pump system pressure. 

(I see you say the pump is off and tank empty)

2" of water over the area of a normal MH sink, say 400mm sq is 50 x 400 x 400 = 8000cc = 8 ltrs that's a lot of water!

Hi IanH,
The tap is over the bowl in the sink, so any water in the system would have gone in the bowl.
I agree it's a lot of water and it was clean and fresh like tap water.
There is also a slight rim around the sink, so we are not sure how the water would get from the work surface behind the cooker and into the sink.
It's a real puzzle?