some one that externally insulated their walls and has patches that appear on the render - the pataches are along the lines of the joints. There was the suspicion that it was a damp pach caused by the morning dew (some thicker render fed into a joint) but more recently I've been wondering whether they are actually dry patches resulting from some thermal bypass (closed loop convection) wicking heat through to the render.
Prob old news? But AFAIK now, pattern staining on joint lines of EWI is a real risk, is due to the joints being more conductive than the blocks, may be due to convection bypass in the joints even if they're fag-paper tight, but may also be due to conductive fixing cement in the joints. We did have fixers who felt free to fill joint gaps with fixing adhesive.
Now we completely fill the EPS block-edge joints by bead(s) of expanding squirty foam as we go, which also serves to keep cement from squishing into the joint zone. We also keep the cement dots/dabs/bead a bit away from the block edges. A bit of air void behind the blocks is gd for insulation, causes no bypass, as long as it's trapped in small cells, not free to convect up and down the wall..