Mystery stains on external wall under concrete sill

[trilby]

A year or so ago I stripped the blown Sandtex masonry paint under our downstairs bay window, revealing some staining that looks like it had come from the concrete window sill. I painted over it with a different masonry paint and within a few weeks the staining had returned.

Whether it was new staining or the old staining somehow showing through, I couldn't tell. However it's only on this one side of the bay.

Attached are some pics. The white gap between the stains is the non-blown Sandtex which I left. It doesn't stain, however I don't just want to repaint with Sandtex only to have it blow again.

If anyone can help identify what's causing the staining and how to stop it, I'd be very grateful.

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[Crystalclear]

Hummm, is that black gloss paint I see before me on the sill?

Before you look at the stain, you'll notice that if you were to get a thin flat screwdriver in the corner of the bay window, where the cable starts to angle downwards, it will push it's way in; whatever it is, black paint that has over time, run off through the gap and into the concrete, or just the concrete 'going off'? You'll need to do one thing. Chip off the damaged area, with a hammer and chisel, let it dry, and concrete it new.

Stone sills normally sit on a thin bed of mortar which over time just goes soggy biscuit like.