does coolant look good or is there an oily film or any discoloration? if no, then it's not exhaust gas.
is it a 1.2 or 1.4 cap?
if it was mine i'd:
a: remove cap, start bike, watch coolant swirl a bit. this just checks waterpump operating properly - let it come up to temp a bit, it shouldn't boil until bike gets pretty hot...and fluid level should not change...
b: take a good look at cap and replace with known good cap - 1.4 works well.
c: triple check coolant level. coolant above the fins will leak out overflow..
d:check for leaks elsewhere - did you see it dripping out the overflow hose? could have come from somewhere else...
if the coolant level was high, it shouldn't be anymore - once it blows out the overflow a bit, it stabilizes...