Dirty Sleeves

In a shop today, as I paid one staff member, her colleague engaged Duncan in conversation.
Seeing him remove all the wrappers from a packet of Fruit Pastilles (chewy sweets) she said, "I hope you're not going to put all those in your mouth at once!" Duncan replied, "Yes I am," probably not hearing or just ignoring the confusing part of the question, "at once".
"Oh my," said she, then noticing a dirty mark on the sleeve of his top, she asked "oh, what's that, is it toothpaste?" Duncan, slightly worried said, "yes toothpaste," then coming to me continued, "I'm very sorry Mummy, I made a mistake. I'm sorry I was cheeky." (There's a bit of Thomas the Tank script in there.)

I assured him he was OK and that he hadn't been cheeky. I know the old lady was trying to be nice, but picking out a stain on a little boy's sleeve, that's entering dangerous territory. When I overheard her initial question, I was almost certain Duncan would tell her it was some boogies. And well it could be.