Troubleshooting 101, Section 2, Part C:
As you progress down the troubleshooting paths without results, you have to get more critical, more aggressive and sometimes repeat things. Examples:
- saying "We have new plumbing, so it can't be that," is not necessarily true. In fact, new plumbing can be just as problematic as old: maybe the plumber was a bit careless, or installed a faulty part, etc. Assume nothing.
- saying "The irrigation guys did that." may not be the end of that path either. Maybe they missed something, were lazy, or an intermittent condition that exacerbates the leak might not have been present while they were testing, etc. Assume nothing.
- neighbors saying "I don't have a leak." You get the idea. If the quick and easy doesn't reveal the problem you're troubleshooting, you have to go back and try again, be more thorough, more critical, whatever it takes.
And Newdude points out another possibility. Maybe the leak only occurs when something is on, which really punches a hole in some of the typical troubleshooting techniques (like the spinning flow indicator, which wouldn't be helpful if something is leaking only while on)...