Green plant detection
This may be kind of a naive question, but I haven't studied all the lander and orbiter science instruments in detail.
Earth resources satellites routinely detect the presence of live vegetation by analyzing various frequencies in the IR band. I'm no expert, but even making the rather enthusiastic assumption that Mars life--if any--even involves photosynthesis, couldn't a Mars orbiter detect such things? Don't we already have a Mars orbiter on the scene with IR capacity?