My house had some of the plumbing replaced with polybutylene 30 years ago. A couple of years ago, I found drips at a couple of elbows under the house. I put a pail under on so I could monitor it by ear.
One was below the kitchen sink. It had been less than a drop a second. A few days ago, it was about ten. It was coming from the floor, at several location.
There's a shelf below the sink, 4" above the floor. The space between is inaccessible. I shut off the hot water at the heater. That slowed it. I shut off the water to the house. Before long, the dripping stopped. I turned the house water back on and got about one drip per second.
That meant the cold pipe was leaking, besides the hot. The pipe fit pretty snugly where they came up through the shelf. There was no moisture on the shelf. I pulled the pipes up about an inch and felt the pipes and connections. I didn't feel any moisture.
This was bizarre. There shouldn't have been any connections between the shelf and the floor, but apparently both pipes were leaking in that space. Most alarming, a beam made from a log, possibly a century ago, had become wet.
In a repair like that, there were a lot of places I could go wrong. I turned on the water long enough to fill some containers for drinking, cooking, and washing. I filled a bucket to flush the toilet. I shut the water off and called a plumber.
I took him to the kitchen to examine the fittings. He replaced the pipes with PEX and the plastic valves with what looked like chrome-plated brass. He said they were better than the plastic valves I'd had.
He found no deterioration in the pipes. He said connectors above the shelf must have leaked. He blew into the pipes but couldn't find a leak. He said anyway, replacing the fittings was good because that plastic got brittle. He tried to demonstrate but couldn't break one.
When he left, I discovered that he'd taken the old pipes. I'd wanted to get to the bottom of the mystery. If the FBI wouldn't investigate the leaks, I would have used my neighbor's compressor.
The next day I found a drip from an elbow he'd put in, under the house. The shelf under the sink had been dry 12 hours after he left, but now there was a puddle. Feeling he wet pipe, I find that the source was the compression connection at the top of the valve. I took it apart and found that the chromed brass that the plastic compression was supposed to seat against, was bumpy. So much for the superiority of metal valves. I got it to seal with plumber's grease.
I can't figure it. The original leaks were much faster. Logically, I knew they had to be above the shelf, but I could find no moisture.