You probably haven't noticed it before because your humidity is generally low where you live. Come to the deep south and this is very common.
Like Patriot said, all internal combustion engines generate water vapor in a humid environment. At startup, the exhaust system is cold and this vapor is instantly returned to the water state until the exhaust system heats to the point to where it will keep it in the gaseous state. But you have to have the water in your atmosphere to begin with and since you are just now noticing it, it's either raining out there or it's about to.
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