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.