coolant coming out overflow

ok so i changed cap. it stopped for 2 months. then now very minor coolant is coming out. i havent added any coolant since april 14 2017. i have a leak by def header so thats why. but the overflow coming out cap what is the cause? like i said its super minor. truck dont overheat, no loss power. nada. just trying to see reasons. 2012 cascadia dd15

 

hmm. i mean it leaks minor out the cap. but isnt there more signs to a gasket bust? jeez its only 600k. so early

 

maybe. i dont know if im crazy. but on thursday i smelt a vinegar kind of smell. did not smell it today. so.....idk if its a head or not.

 

ya i tested it like you said. no bubbles. no white smoke. no power loss. no oil mixing. no engine lights. idk if this normal but i added a pic to see

 

is that little hole on side normal? i think its cap or the tank itself