Before leaving our home for about a month, we closed all sink, toilet, wash-basin, and washing-machine hot and cold water supply valves (all are old "gate valves"). We did not close the main water supply valve. We had left the central heating on to maintain a temperature of 50 degrees inside the house. There was no sub-freezing weather during our absence.

]When we returned, three days ago, we saw that there had been kitchen flooding from under the sink, and there was a strong mildew odor throughout the house. Turning on the central heating cleared the smell from most of the house, but it is still strong under the sink. Water marks on the kitchen floor suggest that water flowed out from under the sink, 18” – 12”, but the floor was dry when we returned. There was a small puddle under the sink but no sign of continued leaking. There has been no leaking since our return, and the sink faucets function properly. I have no idea how long the water flowed or when the leakage occurred. A friend has suggested that one of the under-sink “compression valves” (multi-turn “gate valves” ) failed but then “corrected itself” when a gasket or bushing in the valve swelled or grit in the water stopped the leak. As I have written, there has been no sign, visual or auditory, of resumed leaking since our return.

In addition to attempting to eliminate the mildew odor by heating the house and placing a space heater in front of the sink, we closed the under-sink supply valves and left them shut over night. The next morning, there was no sign of leakage, nor was there leakage after we opened the valves again.

Any suggestions as to possible causes and steps to take now will be greatly appreciated.

You shut off all the valves but didn't drain any pipes? I'll bet that the stem in one of them is a bit worn out, and dripped a bit til the mineral deposits clogged up the leak. There's many other options but that's the main one, in my non-professional experience as a homeowner.

When you turned them all back on, did any drip? Is there a lot of calcification on any? You can tighten the nut behind the handle, or replace the whole valve.

Generally 'vacation mode' is to turn off the main and drain pipes, and to pour special antifreeze in all the traps, including the toilet tanks and bowls, after flushing after the water is off, and scooping as much out as you can out. Then you don't have to have heat at all, or can set it much lower.

A friend has suggested that one of the under-sink “compression valves” (multi-turn “gate valves” ) failed but then “corrected itself” when a gasket or bushing in the valve swelled or grit in the water stopped the leak.

That is so far fetched that if you believe that I have some prime farm in the Everglades I want to sell you.

It is pretty hard to suggest what happened from a distance but I would consider the possibility of a sewer line back. Obviously is you were not there using water, your sewer is not going to back up. But if you are at the bottom or near the bottom of the municipal sewer system and there is a blockage of that line, the sewage backs up just like in your line. When it reaches your line it backs up into your line and into you house.

Granted it is very rare but it does happen. Are you at the bottom of a hill with houses higher than yours? Obviously if this happens, it will happen at the lowest drain drain in your house.
Water supply lines are designed to sustain a lot of pressure. While drain lines are sealed they are not designed to for a lot of pressure. Especially compression connections like where the trap arm under the sink is connected to the piping in the wall.

Check with neighbors to see if they had any unusual sewage problems while you were away. These types of things often occur during storms or heavy rains.

Also consider whether it could have been rain water coming in.

Valves do not turn themselves on and they do not correct themselves.

What I propose may seem pretty far out but consider it and provide more information.