Sensor automatically rremoved from Ring alarm: very distrubing

Today, I was just checking the battery level of devices in Ring alarm app and noticed that one contact sensor was missing. It was totally gone. Vanished. I removed the sensor battery and it was completely dead, the battery tester needle won't even move. So, I replaced the battery but the sensor never showed up in devices. I had to re-scan the QR code and add the sensor as a fresh device. How can this happen? I arm the system daily and if door sensors remove themselves automatically, how can I even trust the Ring system? I don't even know when the sensor removed itself. It is on a perimeter door so purpose of securing the home is defeated. Also, for the drained battery, I never received any notification in the app. I open the app at least once in two days to check if there is any notification, there was none. I get notified about all the cool things I can do and deals and whatnot, but not about a dying battery?