J.S.
We get these from time to time too, come to think of it they are usually on my husbands shirt to. They don't have them going into the wash, but do coming out. I don't know what causes them or why they tend to be on his and not mine. Maybe in a fit of male shirt chivalry it throws itself in front of the unknown stain agent to protect the womens shirts from such filth... or maybe I watch to many movies...