Trouble ID'ing lawn trouble

I just noticed small patches of brown grass on my lawn. Circular in nature and about 4-5inches in diameter. No uniformity to the exact placing of them, though they started around the septic tank which is always prone to drying. They seem to be spreading, too.
It seems to early here for Brown Patch and the closest thing that my lawn book has resembling it is dog urine damage. We don't own dogs and have never seen neighbor's dogs on our property. I have done some digging and I see no evidence of grubs either
Does anyone have any idea of what it could be, or tips to help me ID the problem?
Thanks in advance,
Gary Zone 5/6, Central Ma.
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