I have lots of Aquara sensors (through zigbee2mqtt), and they regularily fall out. The only way to tell if it’s actually 27 degrees outside an early morning is to click the graph and see how long since it was updated. Why can’t this be a core feature for all sensors, give a warning after a (user adjustable, common per integration) timeout?
When I restart HA, Zigbee2mqtt will finally give UI warnings, in that their values are ‘unavailable’, which shows all over the UI. But dead Z-wave sensors have their previous value reloaded, so it looks like they are working.