A few years ago I noticed a sound from under the house, like a pump was running continuously. I went down there, and found that it was. In fact, we have four sump pumps under the house, and three of them had failed, some of them badly, resulting in small amounts of accumulated water at their respective sump pits.
Not long after this, we discovered some water damage because an air conditioner refrigerant line had frozen over, and condensate had accumulated in the back-up location which did not drain well, and ultimately leaked out of the back-up location.
As a result of these events, I purchased some water alarms (cheap, but well reviewed: "The Watchdog" water alarm) and installed them all over the place.
Fast forward to a recent overseas trip. Our house sitter notified us that water alarms had gone off three times. Each one was correct, but she couldn't find an available handyman that would go under the house, since things looked fine when they looked through the vents at the pumps (and I'm sure there was no shortage of other work after a week of heavy rain).
Our water alarms detected:
1. A hole had developed in the middle of a horizontal copper pipe in our hot water recirculation line; this was in the garage and the water alarm was the reason it was noticed before it was a bigger problem.
2. A sump pump which had its float stuck against the side of the sump pit (presumably as it had moved over time). As I had done to other pumps before, I moved it to the side of the pit, and made another mental note to only buy pumps with vertical floats the future (unfortunately these are generally found on higher rated pumps, and I have four pumps on one circuit: I'll probably end up putting in more circuits to solve this after enough pumps have failed and been replaced).
3. A sump pump which had failed badly. This "thermally protected" sump pump was, technically, "thermally protecting" the nearby crawl space from water. That is, it was failing to pump out any water, but was instead activating based on the float switch and then overheating to the point of boiling off the water.
Since we came back, we've had problems with the water alarm by the sump pump which had previously been overheating; it has been giving false alarms, and due to location, it's quite an ordeal to get to and clean or replace. However the number of times these alarms have saved us from long-term water accumulation or water damage makes it definitely worth dealing with occasional false alarms. In fact, I'm so happy with water alarms in general that a future project may be to wire some water alarms into our alarm system, so I wouldn't have to depend on our ability to hear a high pitched alarm going off under the house.