Does anyone have experience with vanishing twin syndrome, and the twin appearing later?

I had my 7w US yesterday but only measured 6w2d and they found a second sac, pictured here. It’s bigger than it looks here as they were focusing on the first, but still it was about half the size of the first sac with the clear fetal pole. They kind of just glazed over the second sac and didn’t measure it, spoke like it was a singleton pregnancy, and are having me come back next week to measure again.

Has anyone had this happen? Is there a chance this baby shows up next week and both twins are alive and well? Or am I grasping at straws? I’m just trying to figure out if I can mourn or not yet. This gray area sucks!!