I live on 6 acres in Oakland County and seeded the 3 acres around my house with a 1/3-1/3-1/3 class A mix 7 years ago. Last year in a drier hard packed area I began to notice these darker green grass circles about the size of a dinner plate in my lawn. When I cut my grass at 4" in the summer they look like divits from a distance because the grass is fine bladed and lays down at that length. Now several have spread into the lush perfect area of my lawn. What is it? In one of the pics you can see where I pulled some grass out of the center. It seemed to pull out blade by blade and not connected in any way. 

Bentgrass? POA of some kind? Any ideas or suggestions would be awesome. Referral to a local expert would be appreciated if not? I can't let it spread into my entire lawn. There are probably close to 100 spots in the one area already.

Thank you.