Actually (It's too late to edit) we do have this curious thing

Aliens - we claim/recognise that statistically the size of the (at least observable, if not entire) universe and number of habitable planets with all the right ingredients for life that there must be life out there... somewhere

But we don't have an ounce of evidence (neither for nor against)

God(s) - we don't have any evidence one way or the other, atheists just say "It's impossible", theists say "It's the only answer", but, as already mentioned, there isn't any actual evidence that can lead us to a conclusion. (This will be misread as an argument for god(s), but it isn't. And even if it were, there's still a massive step between that and the Abrahamic God being the dude)

Which takes me right back to where this started. The supposition that the features of mars are signatures of life, we don't know at this point, all we actually know is... we haven't found anything else that we can say they are.

The reporting of science is causing so much grief (I mentioned it here https://news.ycombinator.com/item?id=45190544 but was voted down for some reason)