I was asked by another adjuster about black marks on drywall ceilings where the studs are. My only experience with this was soot damage from an oil burner or from an actual fire. Have you seen it from something else?

When you say black marks, are you talking about the nail heads or streaks?

It isn't all that uncommon to see spots on the nail heads due to heat/humidity if the texture is too thin.  Fire/smoke will make them come to the surface pretty quick.  Generally see it on painted walls long before textured ceilings.

 

If there wasn't a fire causing this damage, it may be due to ghosting. This is usually caused by excessive use of candles, or possibly from a gas fireplace that isn't venting properly. If this is the case, check with the carrier to see if they provide coverage, as this occurs over an extended period of time.

Yup, if its your typical ghosting phenomena, just dismiss the restoration contractor and call your local exorcist...

Seriously though, I've never heard of that, but know what you're talking about.  See this link about ghosting:  http://en.wikipedia.org/wiki/Ghosting.