Picked up a SS frame at local swap meet (Chicago burb, Harper College). It had plenty of rust spots that looked mostly surface and the guy offered for $10. For that I can play with it or toss it, so I bought. It seemed quite heavy. It was labelled 6066 aluminum -- which was clearly wrong since the rust was definitely steel. (Since looked it up as a bikesdirect Motebecane Fantom Cross UNO -white)
The mystery part: As I carried it around, a little bit of rust seemed to be falling out of the seat tube hole. At home I was able to shake out a lot more --but it did not look like rust. It looked more like very fine sand. Not red but tan/yellow like sand. I cleared out all the vent holes and interior holes between tubes and gently tapped the frame with a rubber mallet. Sand continued to pour out of all the holes. Several pounds of sand poured continuously from all the holes. I'm pretty sure it was sand. Some had a little bit of red color but mostly pure sand color and very fine grained. Fine enough to pour continuously from the tiny holes at the far end of the chain stays. Weird.