There are methods that can be used for finding underground leaks. Some more elaborate than others. If your leak rate is that steady, just isolating each section for a few hours might help to determing which branch is the culprit. As for locating WHERE the leak (or leaks) is, there are some new teqniques working their way around. We recently had a company out to find one of many relatively large leaks in out HHW system's burried piping. Our facilility sits on top of a pretty substantial aquifer, and as such, underground leaks never surface, they drain down - making it very hard to locatie. What they did worked well. They drained the section of pipe, and then injected Helium with a Hydrogen trace gas into the pipe. Then then traveler along the ground, sniffing for the hydrogen (probing into the ground with the sampler, and drilling small holes through the asphalt parking lot to identify the location. They found at least the first of likely many leaks rather quickly with this method. Not really sure what it costs though.
It's the same method they use to check for leaks on high pressure gas mains... lie kteh one that leveled a California subdivision a few years ago. Personally I don't want to be the guy that has to drill the small bor holes TOWARDS a HP gas main.
I don't think it's terribly expensive. You have the gas detection equipment (probably same used for confined space entries and such), then a small diameter boring machine and after that you're just paying a standard hourly rate for the technician. Not cheap, but not terrible I don't think. Our local gas company did it themselves with their local service techs.
Depending on the soil, you shouldn't have to bore. That's the reason for using the hydrogen/helium mix. It will migrate up through even packed soil. The only 'boring' we had to do for ours was to punch through the asphalt.
If your chilled water makeup meter in the plant is showing 3 gpm going in non stop, something is leaking pretty well. Your water chemistry is also shot. You can't afford to let this go on unresolved.
I would isolate the chilled water flow to each air handler one by one to see if that stops the meter spinning. If it does not, start searching the chilled water lines where you can see them, especially where they go into or out of the ground, or enter/exit a building from below ground.
I agree. If you have a coil leaking it may all be going out your condensate drain which would explain why you can't find it. If possible isolate the wings you mentioned first and see if you can narrow it down to one wing then start on the AHUs or fan coils in that wing. Good luck.