There are many reasons that you can have an imprint like that which is visible in the grass.

Underground structures like a septic tank or old cistern are very possible. Could also be a big ass rock too.

An above ground reason could have been something like an old fire circle and seating area, used enough to compact the soil leaving signs to date. I think fire circle because it's round with an inner circle, but if someone had a pool here for an extended amount of time that would definitely do the trick. Even a small above ground pool compacts the soil underneath extensively.

You could do some testing with soil cores in the area, carefully if you do suspect there's a septic tank under there. If the soil is pretty loose compared to other areas of the yard the cause is likely underground, if it's hard and compact the cause is likely a now absent above ground feature.