@ Janleva & All
Why is it so?
This phenomenon is mostly seen in petrol driven engines & that too showing lower reading on odometers.
Water is always produced as a byproduct of just any combustion. When the engine is turned off, the moisture already present in the tail pipe condenses down alongside of the length.
This condensation is encouraged when the car is kept parked for long enough to subject the water vapors to condense under the present climate conditions. When the engine starts, flue gases rush through the exhaust pipe flushing the water out. While any water entraped in differnt crest and trough gets vaporised with due time.
is it Bad / Good?
If your exhaust/tail pipe has a thick layer of Carbon soot stuck to its inner wall, the water present in the flue gases may not be exposed to lower climate conditions thus no water at all eventually. Even if there is some condensation present in the tail pipe, the moment the engine starts, the water stucks/absorbs in rough carbon layers - waiting the temperature to rise to vaporise it to steam / unsaturated vapors.
I think now you know that why it is not apply to diesel driven engines.
O' Forgot one thing. If your coolant / radiator water has started disappearing then it would be beacause that it is ingressing the combustion chambers because of an intake manifold or head gasket leak. If there's no evidence of coolant loss or consumption, you're probably experiencing a "normal" event.
Comment if my philosophy is wrong.