I agree with Haircut. I doubt the heat would be hot enough to melt styrofoam. Styrofoam is a pretty good insulator, so I doubt anything short of a heatgun would do it. At least not without adding some acetone...
I'd put my money on a fault from the factory too. If it's still under warranty, give it a try. The board lady seems to show a few of these faults, but on the deck rather than on the bottom:
boardlady.com/petersjp.htm .
The only way you will really know what happened is when it gets opened up. Melted styrofoam has a particular look to it, and even then the divinycell should keep the shape.
Dale.