Getting ready to do a DIY install of engineered wood in our home. We're the original owners (29 years), concrete foundation on grade/fill, Southern Calif. (Orange County), no extreme weather conditions.

DH took out the carpet in family room and hall, marked the high & low spots, used some Henry's Feather Finish stuff to fill in low spots. All looked fine until we took the carpet out of the dining room where there is a patch of darker concrete that had some white lime residue around it. The carpet pad also had some white residue (no wetness or mold) where it was in contact with this part of concrete. You can see where the concrete is flaking/pitting somewhat around this darker area.

DH shut off the water to the house (for over an hour) and checked the water meter and there's no movement. Then, he cut out a section of concrete to see what's underneath. Below are some pictures. After taking the section of concrete out, we can see a couple layers of metal re bars or whatever they're called. Below the metal bars there is a fine sand type of soil that is damp, not saturated. Over the past few days this sand type stuff has been drying out.We took out most of the sand and can see a thick layer of plastic. No pipes can be seen. There is no oder/smell to this moist sand so hopefully this is not related to the sewer water.

We're thinking this may be coming in from somewhere on the outside perimeter of our home. Ever since we first landscaped, we were cautious to keep the dirt below a certain level because we knew that was a way to avoid water from seeping into the foundation & flooring of homes. We had the carpet replaced once about 12 years ago. When the installers replaced the carpet, we didn't think to look at the foundation. It was replaced quickly and they never mentioned any suspicious spots on the concrete. They were working so fast, I doubt they even paid any attention to the concrete. So, I don't know how long this "spot" has been there. When I say spot, it's really like 3 spots...2 that are close to each other and look like one, and a third smaller one about a foot from the others.

We had to replace our main water line from the house to the street about 10 years ago.. I wonder if that had anything to do with this?

Any one who can give advise, suggestions, or any thoughts?

This first photo shows the entry on the bottom (darker due to skim coat/feather finish stuff). The front door is out of the picture but on the right. Wall to the left of the stain is the kitchen (stove on that wall). Wall at the top has a large window with boxwood plants on the outside. Wall on the right has an air conditioner on the outside.