It is important not to over or under water lawn sod. There may be miniature hills or dips in your lawn that result in over or underwatering of those spots.
sometimes lawn sods turn brown because there dry or the ground there on is compacted to solve this you need to fork over the area and the roots of the new sod will have a chance to take root
i would be saying the if you dont have animals then maybe there a dog somewhere peeping in you lawned,i would bet it the neibor dog or cat that would be my bet