I had my eyes focused to see the hidden image the same way I always do. But this time I was scrolling the picture on an iPad and the next one was coming up and it was directly in focus and the next one too.
This was real incredible - no need to focus again - it is like living in this hidden world.
If you never experienced this I recommend to try it - at least for me it was a wow moment.
I drive Curiosity and that's how I often view our raw downlink images. They aren't stereograms, but the left and right eyes are next to each other on the report, so I just cross my eyes to get the stereo and scroll the page. Works great but despite my trying to spread the good word I think I'm the only rover planner who does this.