Learned something new and surprising about Mars rovers (like Spirit and Opportunity, and the planned European rover ExoMars): that if little green Martians jumped up and down in front of the Rover's cameras we almost certainly wouldn't know it. There are two reasons: firstly, the communications links between the Mars rovers and Earth are intermittent and low-bandwidth, so you can't have a live video stream (webcam) from the Rover to Earth and, secondly, the Rover's onboard cameras have image processing software that is programmed to look for specific things, like interesting rocks. This means that the Rover simply wouldn't 'see' the Martians, they are - in a sense - programmed to expect the expected. Although we are used to seeing the amazing panoramic views from the surface of Mars, these still images are only grabbed infrequently so our Martian would have to be standing in front of the Rover at precisely the moment the image is captured for us to see him (it).
I just spent 2 days with a remarkably interesting group of space scientists (planetary geology, exobiology, etc), space industry and roboticists discussing the science and engineering of Mars sample return missions: i.e. to find, collect and then bring interesting Mars rocks back to Earth. Given the immense cost and technical risk of mounting such a mission it seems to me worth the extra small effort of giving the rover(s) systems that would allow them (and us) to notice unexpected or unusual stuff. An image processing module that, for instance, continuously looks for things in the camera's view that are the wrong colour, or shape, moving in a different way to everything else. The whole point of exploration is that you don't know what's there and, while I'm not suggesting there really are Martians (other than perhaps microbes), it does seem to me that we should engineer systems that allow for the possibility of discovering the unexpected.