Water is a normal waste product of the combustion process. When the engine is warming up, the cold exhaust is enough to condense some water out and cause the remainder to appear as vapour clouds.
This then causes liquid water to build up in the low points of the exhaust - mainly the rear silencer. As the whole system heats up, this then starts to boil off (or just run out), meaning that it can be some time after the engine is fully warm before the steam disappears.
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