Hello Rick

When you dug to look for grubs did you just dig under the dead patches? If so try digging a section that includes both brown and green grass. The link below has a great photo of how to do this.

https://www.canr.msu.edu/news/how_to_choose_and_when_to_apply_grub_control_products_for_your_law

If that technique still doesn't reveal grub activity, then I think the best you can do is give your lawn some TLC, core aerate if you haven't already done that, fertilize, perhaps overseed damaged areas.

If you do see grub activity, the following link will give you some ideas for control.

https://extension.colostate.edu/topic-areas/insects/billbugs-and-white-grubs-5-516/

It is possible that our rainy cool weather a couple of weeks ago created a fungal problem. If so, warmer weather is coming. Make sure the lawn is irrigated appropriately and the problem may resolve on it's own.

Thank you. I dig for grubs every year when this happens, and never see any. But, I apply GrubX since the lawn damage most closely resembles that.
I also spray Fungicide 3, from Garden Safe in case of fungus. Usually win 2-3 weeks it has recovered.
Confusing but it seems to work.