So 99% chance that this is completely mundane, a piece fell off the rover or something...but as Issac Asimov once noted, "The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka” but 'That’s funny...'" This could, maybe, possibly, be such a moment. Keep watching the skies.
Nasa says Mars mystery rock that ‘appeared’ from nowhere is ‘like nothing we’ve seen before’ (The Independent)
A mysterious rock which appeared in front of the Opportunity rover is "like nothing we've ever seen before", according to Mars exploration scientists at Nasa.
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Astronomers noticed the new rock had "appeared" without any explanation on an outcrop which had been empty just days earlier. The rover has been stuck photographing the same region of Mars for more than a month due to bad weather, with scientists at the Jet Propulsion Laboratory (JPL) in California monitoring the images it sends.