Water dump
Would a water dump create an echo traveling at 5 meters per second (about 11 mph) relative to the Orbiter? For that matter, would an impact knocking loose a piece of tile?
If there is a chunk up there, it should still be there. And they would have a pretty good idea exactly where. Are the radar guys looking? If it was a chunk of water ice, would it have ablated to nothing by now in the vacuum? If so, finding an echo a week or so later would make it almost certain to be a structural item.