That's ok, all mine do it, especially when cold. Something to do with hydrogen from the gasoline mixing with oxygen from the air and producing water in the exhaust system. Most of the time it's so hot it'll just be steam and you'll never see it. But when the whole exhaust train is cold the vapor will condense before it reaches the outlet - before it warms up.

Thus the old saying that running a big motor like that for short periods of time is awful, as the same thing happens in the crankcase. Water will sit on top of the heads, sit below the oil etc before it has a chance to really come up to op temp and "boils" off. That magic time? Maybe 20 minutes minimum or so, but repeated and regular 5 minute trips will eventually do it in.

Lots of posts out here about repeated minimum run time damage, although I doubt a few isolated short runs are that bad