Water run on inside rear (boot) screen

Evening all,

Noticed over the past few weeks that there is a evidence of water running on the inside of my rear screen, only slightly may I add.

At first I thought it was external but it never went away when wiped or sprayed, I then noticed that it had run across the heated strips. I'll point out that it is not wet, just signs of a dried trickle. I put this down to condensation but...

No other windows have condensation

It starts dead centre of the window, I know this is the highest point but also it will be in the vicinity of the spray feed (which is slightly offset to the passenger side)

Any one else noticed or experienced similar?

I suppose that I could try to taste it (no taste = condensation, tastes horrible = screenwash)

Will try throw a pic up tomorrow.