I was very interested to read your posts gardeningfanatic. I have had the same brown patches in my lawn, I had looked at the same possible causes. I wondered about ants or drier conditions as it was mainly under a large tree.
There didn't see to be ants in all the patches and it really hadn't been that dry though. Blamed tree roots nearer the surface.
When I read on I saw that you and I have both used the same lawn care company treatment! I have been on holiday and need to go and check out the lawn now my hubby has cut it to see if there is any recovery!