another pic

Here is a link that might be useful:

"Ghost lines" that telegraph the location of framing members are usually the result of airborne contaminants in the house gathering on condensation lines caused by the thermal loss through the framing members during a heating season.

Meaning, your rafter bays are nicely insulated, but the framing members less so. A 10" rafter bay full of insulation can be R30, but a 10" rafter of solid lumber is R10. In a heating climate, the cooler framing members can chill the drywall surface, lowering the effective dew point right in that little micro-area. Humidity/moisture in the house can condense on those lines. It's not "wet drippy" condensation, just that area being ever-so-slightly moist.

That moistness can then capture and trap airborne contaminants, usually from indoor combustion sources; from the fireplace, from burning candles, etc. The shadow lines are an accumulation of pollutants that telegraph the location of the framing members.

mongoct,
Thx for explaining that. I get what your saying, you explained it quite well.

I was so afraid it was going to be something more serious and expensive. We are having the popcorn scraped off and either texture or no texture ( can't decide). I wonder if there is anything to keep it from happening again. Would texture vs no texture ( smooth) make a difference, or maybe if they put up a thicker layer of plaster?