I recently speculated that spacecraft both orbiting and sitting upon Mars may have already detected signs of life. In particular, some spacecraft have detected signs of methane:
In 2004 the European Space Agency probe Mars Express detected the presence of methane in the atmosphere of Mars. Methane can be produced geologically (and Mars is not short on volcanoes), or biologically. (Though media reports of that observation got a bit out of hand.) Either way, this is an important observation and research on the source of this methane is still ongoing.
The existence of methane is ambiguous: Though methane is produced biologically, as I wrote above, it’s also produced geologically (and, in fact, the methane detected on Mars tends to be both localized and emanating from some of the more volcanic regions). It can also be delivered by comets. Given its ubiquity, methane may raise hopes, but in the end turn out to be a poor biomarker. Detecting life elsewhere will require multiple lines of evidence.