The longer it takes to hear the sound or see the leak after you turn it on the farther it is away. If it takes as long as you say it does...
Finding a leak not surfacing on a lateral is very hard. Personally for me what I do is use accoustical equipment when all the heads are shut off at the base of each head.. The probability of the leak to occur at the base of a head and then just flowing into a drain is much much higher then the piping itself.
If I don't hear the sound I try to systemically break down the design of the zone and best guest cut the zone in half to continue on isolating . Sometimes there is no way to figure this out depending on the design.
If you have metal rod you can listen at each head. This would be my first thing to suggest to do .. You could just have a cracked marlex not surfacing and flowing into the drain . recommend to get a cheap phonoscope or stethascope
Try to isolate the piping at a mid way point at least the elimates 50%
I had a awesome job the a few weeks back which I should have taken pictures. A zone had a break somewhere and you could hear after a while a roar of rushing water flowing into the foundation drain. After some bit of time I discovered a failed fitting about 15' from the foundation drain I hear it from INSIDE a corrigated drain pipe middle of the yard. lol