Denounce your own hoax.
People on the Internet are all the time "discovering" things in the probe images and videos. All you need to do is leave something clearly anomalous - say a rough figurine of a hand with four fingers, or anything equally unlikely - where the probe will photograph it.
Then dub the thing something funny - "the Shrub" - and proceed ignoring it.
After a little while, maybe with a gentle prodding, but more likely all by themselves, people will start talking about the Martian Hand.
Point out that it can't possibly be an artifact, because:
- there are no Martians.
- pareidoly is a sufficient explanation for the object's resemblance to a hand.
- it doesn't even resemble a hand all that much.
- and, obviously, there are no Martians and never were.
- it is unlikely, to say the least, for the probe to have landed near such an artifact, unless Mars is covered with them, which we know it isn't.
- the "possibly inhabited" surface of Mars is down several feet of dust in that area. Even if someone had left an artifact back in the day, it would be buried deep, not below a few millimeters of sand, ready to be churned out by the probe's wheels.
- and -- someone who? There are no Martians!
After several weeks of trolling, grudgingly agree to go back to the Shrub to perform some analyses, even if it's clearly any old rock.
But the artifact is no longer there.
No, wait, it is. The old pictures were mislabeled. Sorry for all the excitement, guys. Anyway it's just any old rock. We're drilling now.
Several technicians and analysts that aren't part of the very small clique that prepared the hoax go ballistic and are silenced - the readings make no sense. You patiently explain to them that before going public with anything, given the sensitivity of the situation, many more tests need to be performed. After all, there are no Martians, so the artifact cannot be made of fired clay and cannot contain the organics the analyses show. There must be some simple explanation and we don't want to go the conspiracy route.
After a couple of days outside people start asking, and pestering personnel to know what's going on. Delay. The results will leak anyway.
Several analyses later you convene an internal, secret meeting and explain that the analyses say that the object is artificial. But since there are no Martians and all the previous objections still stand, it must be investigated whether the object could not have been brought from Earth - namely, in the probe. So, a panel of three experts (all members of the conspiracy) is nominated to determine the exact path and provenance of every single bit of the probe.
The whole personnel is requested to keep the whole thing as secret as possible, thus ensuring that everything is leaked to the major media channels within one hour. The world predictably goes mad. Your obdurate refusal to admit the object is Martian makes you an Internet meme. A cartoon with the ESA's spokespersons mashed in a lion's mouth while confidently declaring "See? There are no lions! This is just pareidoly and wishful thinking!" makes the cover of Time.
Your panel comes out with possible ways someone might have smuggled the artifact all the way to Mars, each more farfetched than the last (one of them is true, though), and gets lampooned and ridiculed. The panel leader gets interviewed and describes the wildly improbable way someone might have equipped the probe with the Martian Hand and devised a way of deploying it on command.
Q. "And could all this have been done without anyone noticing?"
A. "Well, nobody would expect anyone to try such a stunt, so-"
Q. "You mean that I could enter ESA's labs and add a little something to your next probe, just like that?"
A. "No, no, of course not! But, well - perhaps - if you had collaborators inside -"
Q. "Professor, I took the liberty of asking your colleagues how this could be accomplished. Apparently you'd need the collaboration of nearly everyone. The probe has been weighted, shaken, tested, disassembled and reassembled several times, as you very well know. Its weight is known and the artifact doesn't appear on the manifest."
A. "The weight has some tolerances that -"
Q. "That would have sent the mission anywhere but Mars. That weight must be accounted for in terms of fuel, again as you very well know."
A. "When the impossible has been ruled out, what remains -"
Q. "Ah, but who says that the Martian Hand is impossible? Is it not true, Professor, that you're a lifelong member of several 'skeptic' associations who staunchly deny the existence of extraterrestrial life?"
A. "We claim nothing of the sort! It's simply that there's no proof that-"
Q. "And now the proof might be here, and you won't accept it because you never had proof before? How do we prime this pump, Professor? Somewhere we'll have to start accepting proofs, won't we?"
As a nice touch, two "investigators" are denounced as declaring to be willing to fake smuggling the Martian Hand and accusing themselves, before allowing this farce to go further. After that, any clue suggesting that the Martian Hand might actually have been smuggled is automatically suspect.
The last line of defense of the Hand Skeptics is "If that's a hand, well then where is everything else? Where is the arm, but more - where are the ruins, the streets, the whatever else a civilization must have left behind? You really want us to believe that there's only a hand left, and that by chance we happened right smack on top of it? Come on!"
At that point, a grassroot movement emerges claiming for a manned mission to Mars armed with shovels.