Small Puff of smoke at start up -- Normal?

[GSRP]

I only drive my Boxster on the weekends -- so it sits during the week. I've noticed the last few times I start the engine, that a small white bit of smoke appears when I start the car. It does not continue after that.

It's a 1999 with a rebuilt factory engine with about 12,000 miles on it. The engine oil is full (checked via dip stick and gauage on the dash).

Is there any reason to be concerned -- or is this a result of sitting idle for long periods of time?

Thanks to all.

-gene