Heavily inspired by this thread, I offer the following story and question:
I was recently snorkeling in the coral reef of Half Moon Bay, Roatan, Honduras. About ten minutes after I got out of the water (I had been in for about an hour), I happened to notice three identical marks on me: one on the back of my right hand and two more on the back of my right forearm. Each mark consisted of a perfect red circle about one-quarter inch in diameter with a darker red dot (small, ~0.5 mm) in the exact center. (I have white skin, if that is relevant.) The marks had very little (if any?) height to them – that is, I could not feel the marks with my other hand. The red faded away over about three days. I never felt any pain at all. It these had been on my back, I might never have even known…
While in the water, I made a point not to touch anything (mostly as a precaution against damaging the coral and whatnot), and I thought I was successful, but it seems not.