That's the only part that bugs me, honestly. I'm not one of these "icon theft" whiners that likes to claim they're reporting anyone to LJ Abuse or whatever (as if they could do anything anyway!). No, I'm happy that anyone likes my work (even if they didn't know it was my work), and wants to use it. I did drop them a comment mentioning the situation, though, saying that I was not angry at it's use, no worries, feel free, and merely asking how they'd found the icon.
Incidently, that's part of why I add names to almost every icon I do these days. Even though *I* may not mind, I know a lot of people get touchy about their icons being "stolen", especially if it was something they asked to be made for them; it's like their design and their icon, and they like the uniqueness of it being theirs...so adding a name pretty much ensures that it won't be snagged by anyone else (unless they want to put some serious graphic work/time into it, and if they can do that, they probably make their own already).
And, no, I'm not mentioning any names, no need to do so. I'm not here to "out" anyone for some imagined wrongdoing. Just mentioning my day, eh?