its probably normal...
an engine needs fuel to start, and a cold started NON-computer engine cannot possibly burn all the fuel cleanly that may be in the cylinders upon initial cold startup, and a very very small amount of fuel residue can leave a noticable oil rainbow on the water in certain conditions... as much as it may always happen, we hardly ever notice it, and gas engines are worse than diesels for this.
the other thing is, there could have been some exhaust soot that has broken loose and was discharged....which often happens with an engine that hasnt been run in quite awhile...