Today I had time to test our ideas above. First I did a big circle around the offending area while the sheet was on the bed. Sheets were carried to the basement to the “treatment” area. While the sheet was on the table I noticed a small mark near the original one. I didn’t circle it. Treated both marks. The little one disappeared quickly, but I wasn’t sure about the larger, circled one. So, safety pin time. Into the washer and with the safety pin, it was easy to find. Marking pencil was gone. Stain was gone. Pulled the safety pin and into the dryer.
I think @SuiteInSeattle and I have come up with a routine that can work!