Mystery damp stains on wooden windowsill

Hi everyone,

I am looking for ideas from you helpful people as I am really stumped.

A few months ago I noticed water stains (dry to the touch) all along the inside of my wooden windowsill, in a thin line at the right angle where the sill meets the frame. I cleaned the marks and then tried repeatedly throwing water at the area from outside (sticking my hand out of the opposite side each time). No water came in, so I assumed the problem might be condensation or the result of rain coming in when it was open at some point.

A few weeks later I noticed the same mark was back again, and was bigger, despite the window remaining shut the whole time. I repeated the test and still nothing, so I decided to just be vigilant in terms of condensation.

Well, around a month later I've just found that the marks have reappeared! In this time I've checked several times a week, especially after heavy rain, and never noticed any dampness or condensation.

This is really frustrating me, and I am starting to worry that water is somehow getting into my walls and other places. The window is a dormer on the third floor (typical...) and was re-painted and treated (by a very reputable local company) earlier this year. I have stuck a mirror outside and had a look, but can't see any obvious problems with flashing or anything like that. Does anyone have any ideas, or any recommendations for what sort of company I could contact to investigate this for me?

Many thanks,

Laura