You are not going to notice the drip. That is the whole problem. Mom's house is quiet during the afternoon, you are on the other side of town with the kids in the back seat, and a slow leak under the kitchen sink can run for days before anyone sees the cabinet floor go soft. A water leak sensor is a small, cheap-to-own fix for that, and it is one of the few installs where the hardware is the easy part. The hard part is deciding where it goes, who gets the alert, and whether that person can actually do something with it.
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Pick the low spot, not the most visible one
Water does not travel uphill, so the sensor belongs where water would pool first. For most houses that means the base of the kitchen sink cabinet, behind the dishwasher or next to the disposal, and the floor beside the water heater or the washing machine. If the house has a basement, put one on the floor near the lowest corner and one near the water heater, since those two spots catch most of the damage. Check the sensor's sensing points on the underside before you set it down. The pads need to sit flat on the floor, not on a raised lip or a mat, or the sensor will read a leak that is not there or miss one that is.
Pair it before you stick it down
Most sensors pair through a phone app, and this is the step that quietly decides who owns the alert. Before you peel the backing, ask yourself three questions. Whose phone is the app running on? Who created the account, and does Mom have the login? Does the sensor need a hub plugged in somewhere, and if so, is there an outlet within reach of the Wi-Fi? Write the answers on a sticky note and put it inside the cabinet door, where the next person to open that door will find it. That note is the only record of how the thing works when you are not the one standing in the kitchen.
Test the alarm, not just the app
Set the sensor in its spot, wait for the app to say it is connected, then wet a paper towel and touch it to the sensing pads. You want to see two things. The first is that the phone buzzes. The second is that the sound from the sensor itself carries to the room where someone would actually be sitting. A small alarm under a cabinet is easy to miss from the living room, so stand where Mom usually sits and listen. If you cannot hear it from there, add a second sensor or pick a louder model before you walk away.
Decide who answers the alert
This is the step most people skip, and it is the one that matters. An alert that goes only to your phone, three hours away, is still an alert, but it is not a plan. Pick the person who will check the house within the hour, whether that is a neighbor with a key, a hired helper who is already scheduled for that window, or a family member who lives close. Then go one step further and find the main water shutoff together, once, while the water is off and nobody is in a hurry. Mark it with tape or a tag. Knowing where the valve is turns a flood into a bad afternoon.
Set a battery check and keep the paper
Put the battery type and the replacement date in your calendar, the same way you would track a filter. Keep the box, the manual, and the app login in one folder at your house, not hers, so the file is there when you need to re-pair a sensor after a battery swap or a router change. A quick check every few months, where you test the pads with a damp towel and confirm the phone still buzzes, is enough to keep it working without turning it into a chore.
When the alarm goes off
If the phone buzzes and nobody is nearby, the person on your list goes first. If that person cannot get there, call the plumber or the service line you already keep on file, and do not wait to see if it stops. Do not try to fix the pipe yourself in the middle of a leak unless you already know how, and do not make the call about repairs from the car. A sensor is meant to buy you time to get the right person into the room. If you want a short list of local help to put on that same sticky note, the Local Help in St. Louis page is a good place to start.