For what it's worth mine does this too. There's often a trace of dried coolant on the outside of the bottle. A cheap new cap makes no difference, whereas an expensive new cap cures it for a few months sometimes. It sloshes around as you drive along and the pressure forces it out of the breather. The breather is supposed to let it breathe after all. Obviously, I remain alert to other symptoms of a blown gasket such as steam in the exhaust, oil in the water, water in the oil, overheating, traces of exhaust gas in the water et cetera. but it's been like this for 100,000 miles in my ownership and doesn't seem to be getting worse. Mine's not remapped, as it goes faster than all UK limits in standard tune, but I do drive it flat out.