Understandable! I have a couple zones set up (outside--I need more than one to reduce false motion from shifting sunlight, reflections from cars, and whatever else the sensors think might be people; and in entry spaces where I use the sensors for HSM and don't want something similar during the day tripping the "alarm" unnecessarily). I'm also worried about one getting "stuck" and the best solution I have right now is using Device Watchdog, a fantastic community app, to make sure the sensors have "checked in" within a reasonably recent timeframe. (I use it with the activity-monitoring feature; there is also one that can look at battery level, but if you haven't found that to be unreliable yet, you probably will soon.)
I only have Device Watchdog check my things once a day at a time when I'm usually home. Therefore, it won't necessarily immediately catch something that "falls off" or stops working during the day (my notification comes in the evening), for example. But it's good enough for me and I don't see any easy way to solve that problem unless I use a short interval and multiple checks per day, a bit more effort than I wanted to go through. 
I have several "child apps" set up with different intervals depending on how long I'm willing to wait without device activity before I think there's a problem (most of my Zigbee motion sensors also do temperature and therefore tend to report in at least every hour or two, so I have the interval for those set to just a few hours; some Z-Wave devices I have don't report anything unless they're used, so their interval is a couple days--you have to have a feel for your devices and your comfort level and tolerance for false positives.) Anyway, something worth checking out if you haven't yet!
