The above image shows methane plumes detected on Mars. That doesn't sound that interesting until you put it together with the fact that more than 90% of methane around us is biological. Only minor geological or inorganic sources for methane are known. And the plumes are biggest in the summer, when microbes would be photosynthesizing on solar power. (Or maybe it's a geological process driven by the sun?)
Could this be the sign of microbes on Mars? Keep in mind that there's significant exchange of rocks between ourselves and Mars, and maybe some spore or something got ejected, floated over, and took hold.
Not to mention, the methane goes away quickly, and we're not sure how the methane could be oxidized that quickly. Mystery upon mystery.
Whatever it is, unless we can come up with a convincing non-biological explanation for it (and its timing), it's a interesting sign. I'd love to study martian biochemistry.