Update, FWIW. I went to university clinic. I think the person who dealt with me is grad student. I couldn't remember exactly which "molar" was declared to have small cavity in top crevice. She checked for cavities in all molar tops with her "explorer". In one tooth top, she pressed repeatedly. But decided she couldn't find a cavity in that one. She said it could be easy for one person to find a cavity and another person would not find it.