I just pulled up my carpet & found some hairline cracks (less than 1/16" wide) in the slab. (Floating slab on soggy Louisiana soil; no pilings that I'm aware.) I followed the cracks and each one terminated at a small piece of plywood (4"x4") nailed to the slab. Is that some sort of foundation repair? I've had the house (& carpet) for 12 years; so, it's obviously older work. And, no, nothing was revealed to me at time of purchase. And, no, I had never heard of a home inspection at the time.

I have always had problems with a couple of sticky doors when it rains & some ugly cracks have appeared; but, when I got 4 different foundation experts out five years ago, they said my house was level within fractions of an inch ... general consensus was that the cracks were from bus and truck traffic & the $20,000 foundation repair that half of them recommended was only "preventative" (the other half recommended doing nothing, which I gladly did). Surprisingly, none of them considered the 3 oak trees to be a problem for anything other than the sidewalk!

I'm just trying to figure out what I'm looking at and if I need to re-explore the foundation repairs to address my sticky doors and cracked walls.

Hairline cracks in a slab are not uncommon. One of the best concrete finishers I know says, "It's gauranteed tobe hard, to crack, and to be burglarproof"

But I find it strange that these lead to 4x4 ply squares nailed to the floor. Are these squares set flush down into the concrete? It's hard to imagine they were under the carpet sticking up all this time and not noticed so I assume they were.
Sometimes the method used to screed wet concrete level, is to drive a wood peg everyu so often and level the tops of all them within the form for the slab. The concrete is floated to the tops of them and then the pegs are pulled as the finish crew does it's job.
I imagine that these pegs werre a ;ittle low so they tacked another scrap of plywood to the tops but failed to pull them out in the finishing process.

If they set up above the finished slab, I'm aws mystified as you

Thanks for the reply; I guess we'll both remain baffled. The squares are not sunk into the concrete. The plywood is thin, so it wasn't real noticeable under the carpet & pad. I've found 3 of them throughout the house (so far) & the common denominator is they are all at the end of these long, narrow cracks.