Moving White DOT near Phoenix Sky Harbor Int. AirportI was taking a quick Smoke break at work near 48th St. and Southern Ave. just a couple of miles South East of Phoenix SKY HARBOR Int Airport. There was a Cloud Bank, just North and West of the Airport, highlighted by the morning Sun in Dark and White Clouds. I could easily see Jets taking off from the Airport and was able to judge this object as just a few miles further North.The best way to describe this is: A SMALL WHITE, but nonreflecting DOT. Moving East to West. I caught it as it went in front (towards me) of a Cloud that was Dark. It was moving at a pretty good rate, but I know that even if it was a small plane like a Piper or Cessna, or a Traffic Reporting Plane, I would be able to make out more than a small WHITE DOT. (Kind of like WHITE OUT before it dries and has a Shine to it).It went into the Cloud Bank, and though on it's visible trajectery, it should have reappeared in the Break between the Clouds, it did not. I have lived and worked within a few miles of Sky Harbor Int. Airport for nearly 18 yrs. And this was the stangest thing I've seen.