I would seriously question if this happens all the time, these days. The whole copyright thing is way behind the digital and internet revolution. Look at what the Prince case did for transformation copyright fair use.
I'm an artist as well. I think this can happen whenever anyone sees your art anywhere online. They can copy it. They won't tell you about it. They might copy it really well. And they might copy not just your technical style, but what your art says and how it says it.