One spectacularly unambiguous item:
NASA/JPL Image, Spirit probe SOL 1402
Item easily seen in photo, not close to limit of resolution:
This one is 100% unambiguous. Either NASA is making this stuff up, or Mars used to be inhabited, no third option possible. Item is in a rocky and rough area some distance from the camera and almost certainly inaccessible to the Spirit probe and hence could not be viewed as something which the probe itself left in the area.