Don't dry your hands on Strangers!

So yesterday me and Noah went to the Central branch Library to learn origami even though it was ridiculously cold and the wind almost blew us over. It was actually pretty crowded there. We finished up and I took Noah to the children's room where he went to the bathroom and like Daniel Tiger, flushed and washed and we were on our way. He even dried his hands at the hand dryer. But I guess in the moment my eyes left him, something interesting happened.
"Excuse me sir?" said a woman, a mother, older than me, but I'm not sure by how much, "I just wanted to let you know that as your son was coming out he dried his hands..." and she gestured wiping her own hands on herself and so I assumed that the rest of the sentence was that he wiped his hands on his pants. Which is something I might often do, you know, to save paper towels and the environment and all that.
"Okay." I said, now looking for my son with my eyes.
"I'm not sure if that's something he does but if it was a behavior my child did I would want to know about it and that's why I'm telling you." Her face was very serious. More serious than one would expect for someone who had just seen a kid wipe his hands on his pants so I started to think maybe I was missing something.
"Wait what?"
"Your son.."
"Okay."
"Was coming out of the bathroom.."
"uh huh."
"and he wiped his hands.."
I nodded.
"On me." She said
I sort of laughed but maybe that's not appropriate but what does one say in a situation like that.
"Sorry."
Now I had to find Noah and ask him why he did that. He does wipe his hands on me... but that's different. i'm not a stranger. And can you imagine doing that as an older kid? His only response to why he did this was, "I don't know." And then embarrassment and trying to change the subject.