I'm betting you've got a pot or bowl that matches the ring diameter perfectly. Try a white eraser please.

Thanks - as in a pencil eraser?

Uh-oh. Wait a minute. On second glance, those look like vacuum pod marks. Measure the diameter of the rings please. Go to your fabricator's shop and measure the black vacuum pods he uses to handle slabs or hold them on the CNC machine. That's what's gonna be the same diameter. Measure the distance between the rings and the distance between his pods too, although the pods may be adjustable and may have changed.

They may come out, they may not, but at least you'll know who's responsible. Their mysterious sudden appearance points to pod marks too, plus their location on the slabs and their lining up so nicely. You certainly didn't set three giant bowls next to each other.

Tell them to leave the protective film on the estone or fab the sheets upside down and quit cleaning their pods with acetone which breaks down the rubber enough that it gets sucked into the top.

It's possible that your fabricator's supplier made the pod marks, but he's on the hook just the same.

This eraser please. Or denatured alcohol and a Magic Eraser. Stone slurry and Dawn dish soap, but I'd make them do that as a last resort.

6 months after install they just appear?
Should we try eraser first to see if go away?

"6 months after install they just appear?"

Sure. The acetone they used to clean the pods broke down the black rubber pods. The vacuum sucked it into the estone where it disappeared. It's leaching out now.

You want to do things that can't make things worse. No Bar Keeper's Friend. Don't scrub too hard. You probably shouldn't do anything, because these guys are going to be looking for every reason under the sun to make this not their fault.

If they pull out the "Sorry, insurance doesn't allow customers in the shop to measure pods" line of crap, the pods and the distance between them align perfectly with your marks, believe me.

Re 6 months...change in humidity?

That's as good a guess as any.

If they come out, they aren't coming back. They may not come out, they may come out well enough that a buyer's home inspector may never find them, but the homeowner may always be able to see them. They may come out completely.