My daughter's 2004 Prius developed an engine noise and rough running that turned out, according to the dealer, to be a spark plug that broke off inside the engine. They replaced the plug, but don't know if the pieces are still banging around in there and whether they will cause engine damage. I haven't seen the old plug yet. They were replaced a few thousand miles ago with genuine Toyota plugs. Any ideas on this? Thanks, Gregg.