Extremely local news update.
The plumber was here (cha-ching $75 travel charge) to check the water running down the outside of my house and other things (when you get him here for one thing, you make that $75 work for you).
The reason I could not find the leak by listening to the pipes is because the leak is in the reverse osmosis unit. Since that belongs to the village he couldn’t work on it.
I called the village clerk, who called the company the village uses to service the units, and they talked to the plumber to instruct him how to cut off the water to it.
The plumber will be back tomorrow with parts needed for the other jobs (no additional charge) and the mechanic for the r/o unit will also be here tomorrow.
In the meantime, we have a Britta water filter pitcher for just such emergencies.