Oh, and here’s a thread from last week from one community member who had devices go missing from the app, but available through the web UI, just as an example:
Most of the devices and routines gone
That thread reminded me there is a relatively new option in the app that I myself have never used, but that allows you to hide some devices. Apparently some people have found that it turns on by itself on occasion. So you should definitely check if that got turned on somehow. 
Choose “devices“ from the main menu in the app, and then “manage devices to use.“ see if the sensor shows up there, and if it does, does it have a blue checkmark.
