If life's signature cannot be detected from the mouth of a cavern it will require drilling into or driving into the cave. Finding a cave with a drivable entrance seems possible, but requires a level of driving around and checking that is not likely to happen before there are people who can walk in where their vehicle could not go. We might get lucky and find an accessible cave entrance on our first few tries within driving distance for our current crop of vehicles. Then there is the problem of communication. With the amount of iron in Martian soil, it seems unlikely that our vehicle could communicate from within a cave. It would have to move into the cave under its own programs and extricate itself from any problem in the cave without external assistance and then report its findings on exit from the cave.
Without some major upgrades to robotic explorers, we shall have to wait for humans to arrive with testing equipment adequate to detect signs of life, past or present. Of course, that will raise significant further doubts about what is native and what is contamination. Can you imagine trying to keep everything surgically clean after X months in a tin can flying through space and then being largely limited to that same tin can once reaching Mars. If it is technically possible, can you imagine how much those people would want to escape into a cave with a bit of room, contamination or no. They would probably need to pick another cave for testing after they had a chance to clean themselves up in the first cave.
That was a little tongue in cheek, but not by far. If they are going to be exposed for another 600+ days at the very least, they will need to spend a big chunk of that under some significant shielding, either piled up Mars dirt, or in one of those many caves. Anybody not planning for that isn't serious about actually living on Mars for one Mars year or a lifetime. Since the caves don't require any construction, I would vote for them.