Hi there
Working on aircraft engines, I would say first of all aircraft exhaust pipes are very short just over a meter or so long from the last cylinder compared to cars and as a result any heat is dissapated very quicky after engine shut down, so there is no place for condensation to really collect as you have in a car system with silencer boxes. If there was any vapour it would be hard to see as with aircraft engines(horizontal apposed engines) tend to collect oil in the barrels and on start up you usually see alot of white smoke from the exhaust as the engine starts and the exhaust pipe gets hot very quickly, if you look on the ground you will mostly see traces of oil after the aircraft has moved away. Car exhausts take much longer to heat up and as a result water collected is slowly explelled as the pipes heat up.
Hope that helps