Recently I have just noticed a broken circle like patch on the ceiling in my spare room. The inside of the circle is still the ceiling colour but the edging is brownish in colour which I thought indicated some form of damp/wetness. The area has not "welled" up and isnt flaking.
I got into the attic last night and checked the area - none of the wooden bars are damp and the insulation is bone dry.
I had exactly the same problem with my 3 year old house. The builder took up the floor above the leak - there were water pipes there but it was all bone dry. Eventually turned out there was an intermitent leak in the pipe but 3 metres away from the stain. The drip ran along the pipe until it found a low point - above the stain but it was too small an amount to cause bowing.
I would therefore check all along the pipe run if possible!