Well if its not your dog(you can use tomato juice to stop this,that is what I have been told but how?do you make the dog drink it or do you pour it on the lawn) it might be local wildlife or a fungi ,hopefully not honeyfungus.
How big are the patches and how yellow? What is the growth of the grass like? Is there any thatch, weeds or moss (sorry not casting aspersions on your lawn ) and are you absolutely positive that no bitch has been able to pee on your lawn lately?
Do you have foxes visiting your garden? A vixen's urine will cause yellow patching too.
If the lawn is fenced off then it's unlikely that its your dog... I have a bitch that uses the lawn toilet but she loves tomatoes so she gets one daily. I agree with the Dove it could well be foxes.
Check out this onfo: http://www.qc.ec.gc.ca/ecotrucs/solutionsvertes/disorders.htm