Live rock is amazing - creatures keep coming out even years after you put the rock in the tank! I suppose some are so small it takes a long time for them to grow large enough for you to see, and other arrive in the form of eggs.
I actually put got three to PUT in my tanks to see how they would fair. Unfortunately, two died within two weeks and the remaining one only lived a few months. Haggs tried a couple in his tank as well with better success but did note that the glass was scratched where the larger one stayed for a long period of time (he discovered the scratched at the bottom of the tank after the critter died). This being the case, I suspect they could be a problem with acrylic tanks. Mine, However, would not go on the clear section of the tank, only on the black sides and tubing.
I remember learning about these guys in my Marine Biology course. If I remeber correctly they make a depression in a rock that becomes their home, they only leave their spot to eat. They have some kind of homing device where they always find their spot again, really cool.
It came from live rock in a 7 year old tank of an experienced reef keeper. At 2-2.5", I imagine the chiton has been in a tank environment for quite some time. I have another piece of live rock that came from the same tank with more than 100 feather dusters on it. I have seen other pieces on which there are 1-2mm corals.