I put Zwave window sensors on all windows at work (40 of them). These devices have two AAA cells. One of the gent's window sensors used to "die" far quicker than any of the others and eventually stopped working. I should explain: "gent's" := men's toilet.

The sensors are quite large and simple and the gent's windows tend to be left open more often than the other windows. One of the two gent's sit down toilets is generally preferred to the other for very minor reasons but it is preferred.

So, the battery terminals were getting slightly corroded on that window sensor because it was open more often to the outside environment.

I've rubbed a bit of silicone sealant into the crack between the two parts of the sensor and expect that it will survive better now.