It could very well be little grease spots from eating or cooking. Or it could be residue from liquid fabric softener or detergent that doesn't wash all the way out. I don't use fabric softener or liquid detergent, so I know mine comes from grease.
I use Shout spray or the bottle with the brush. Put a little on there and re-wash. I have found with Shout you can pre-treat and it doesn't fade your clothes before you wash them.
I used to use Spray-and-Wash for stains, but I ended up ruining too many clothes because I would pre-treat. By the time I got it in the wash, the Spray-and-Wash had faded the fabric on the area I treated. I have not had this problem with Shout.