A few questions:

Were they wearing shorts and short sleeves or tank tops?
Did they purchase anything from the supermarket?

I'm thinking that they accidentally rubbed some syrup or glue or fruit drippings from fresh produce which was probably clear and it attracted stuff and darkened and finally showed up.

I've had something similar happen to me and that is all I could figure it to be. It was very difficult to remove, sort of reminded me of the glue on the back of item stickers from stores. My Dad thought it was tree sap though I hadn't been around any trees that day. But it was summer and I was dressed in shorts and tank and they showed up on an arm and above the knee, and I had gone to the super market for my Mom, so that is all I could figure, that it was something from the store.

Just my 2 cents,

STM