I suspect that the ocean is still there but that it’s frozen and covered with a layer of dust.
I was just going to ask what they think happened to the water. Your theory makes a great deal of sense.
Unless it's buried deep, one of our Rovers would have found it.
Imagine this scenario, though:
We dig deep in Mars with a new Rover or in person, and we DO discover a shallow, frozen 'ocean'.
We take back a slice for analysis on Earth, and discover tiny microorganisms that are COMPLETELY alien to our biology. Maybe they are silicon- and not carbon-based. But they are so hardy, due to the harsh conditions of Mars, that when they inevitably get released into our world, they conquer it in a matter of months, killing all of us and any carbon-based life whatsoever.
I officially copyright this plotline. I shall write a short story about this scenario. See you all soon.