Mars
The key to it, might be having a probe that can drill down some way, with a bit of luck with where it landed.
A mobile one with a decent drill seems big and expensive, which NASA does not do at least for Mars missions anymore, which is probably sensible.
If a sustained programme of Mars missions had carried on after the Vikings, without the 20 year hiatus (not counting the failure of the 1992 orbiter), we might well have found evidence for low level life, past or maybe present.
My gut feeling is though, is that it will take a decent manned mission to find it if it's there, along the lines of Zubrin's 'Mars Direct' with it's long surface stay times.
I cannot help recalling that one of the holy grail's for scientists studying the Moon in the late 60's, was to get a piece of primodial rock.
The early, understandably easier landing sites for the early Apollos, were not going to do this.
Come Apollo 15, the first J mission with 3 days on the surface, to an exceptionally interesting site, with the rover, with more heavily trained in geology crews, a piece of this rock was found.
Even a robot rover built now, would likely not have seen it, from the same landing site as Apollo 15.
Certainly if a probe did find compelling evidence, a Mars Direct style mission could get a lot of support.
And have a set timetable rather than a fuzzy 'commitment' after some far off date.