Leak sensors are only as good as what you do with them. We have many leak sensors around the house due to my DH's insistance. Half the time, we don't even know where the alarm is going and we are chasing them! If you are not home, and there is a catastrophic leak, the sensor does no good. Where it has been quite useful. We just have a cheap battery operated on in the hot water heater catch pan in our rental condo. We change out the battery once per year. If the hot water tank leaks, water will go to the person below, which will be disasterous. Recently, it went off and there was a pin hole leak. We replaced the tank.

Google is a good place to find info, but this site is better for finding the experiences of other homeowners. Unfortunately, the home we're buying doesn't have an alarm system to use for this purpose.

As you said in your ORIGINAL post a flashing is missing. They never put the correct flashing in. As JDS posted above in his last picture, that is what you need for a flashing. Instead they put in that monstrosity that does not help. ALSO, there is a valley that terminates near the problem point. And the roof slopes towards the house. The entire area needs the shingles pulled and ice and water shield applied BEFORE the flashing. That ice and water shield should terminate at least a foot up the wall, then seal the top edge with the flashing tape. NOT felt paper, ice and water shield. Everything they have attempted needs to be pulled, a lot more shingles pulled at least 3 feet up the valley, ice and water shield applied to the whole area and affected wall, flashing applied and taped, then finally reshingle and siding done. If he cannot do this right he needs to hire a roofer that knows what they are doing!!!! Obviously the roof design is bad for drainage in that corner but it is manageable with the correct materials and knowledge.

if the failure happens when I am not around (vacation) then the above recommendation can't be applied Having a main valve that is failing at the same time with a pipe or manifold valve failure is very unlikely. The main valve will make sure that even if the system fails downstream of that valve the system will be off excepting the short time when the watering program kicks in. I can live with that risk which is far smaller than having the irrigation system all the time loaded and under pressure (when there is no main valve for the system)