Ceiling spots

I have dark spots, (not too dark), on my ceiling in some of my rooms. I have white "popcorn" ceilings, and up by where the wall meets the ceiling I have darker , almost like dirt spots. At first I thought they were from the hot water heat registers and radiating up, but since I noticed some toward the middle of my ceiling also..I'm thinking, they are sections of my attic that don't have enough insulation layed down.
Is this a good assumptions??
and what would be my next course of action??
Thanks
Barry

Without seeing them, it's hard to say. Could be a leak, could be mold, could be mold caused by a leak.

If there's no vapor barrier or there's no insulation in that area, it could be condensation wetting the area. Probably lots of other things.

One co-worker ended up removing 50# of wax and honey from a hive that ended up in the attic. The honey stained the ceiling. Could be something that died in the attic. You'd need to go up there and poke around.