For all who are still following, I think we found a great solution. Bleach seems to work like a charm when you want to remove permanent marker from concrete. Our house was built about 15 years ago, so I imagine the marker has been there for that long and was deep within multiple layers since concrete is porous. We tried bleach this morning and there is still a very slight discoloration where the marker was, but it’s barely noticeable and I imagine once we stain, it will be gone. Hope that helps anyone searching for an answer to permanent marker on concrete!