Intermittent lavatory leak cause determined...
...now to figure out a fix.
The stuff stored under the bathroom sink has been wet intermittently when we use the Sunstar, since we bought it. There is evidence that it was leaking before our purchase.
I first determined that it was from the underside of the hot faucet, just an occasional drip, sometimes nothing, sometimes enough to get things wet.
I disassembled and reassembled the cartridge without finding anything obvious.
Finally, while using the moho as a guest suite last weekend, I noticed it and the light, figuratively, went on. It happened after turning on the water heater. Apparently, there is a water check valve on the water heater cold inlet. Heating the water causes it to expand, raising the pressure, until it finds a place to push open enough to leak.
The WH has been off since yesterday, and the drip was gone. I verified the diagnosis today by firing up the water heater and checking the leak. It was dripping after the water got hot.
Now to figure out the fix.
I've been aware of this happening before. In my parents' condo development, there was a history of above the ceiling leaks in the copper fire sprinkler lines that took a long time (months or a year, maybe) to manifest. There is a check valve on the inlet to the line. Over time, thermal cycling let a small amount of water, maybe a drop or 2 a day, past the check valve when it was cold, Then, the heat in the attic expanded the water, slowly breaking the weakest solder joint and pushing it apart until it leaked.
The builders fixed it by installing a pressure relief on the inspector's test valve location.
2014 Sunstar 31KE
1988 Suncruiser 31 RQ
1968 Travco 210