Thank you for the detailed informations. 
- Let the mqtt thing go offline generalizes the solution.
- Threshold Alert and Open Reminder [4.0.0.0;4.9.9.9] simplifies definition of Items and makes alert comfortable
- Service Status Standalone widget enables a useful information in overview tab.
Great!
Offline Things Display is another option. But I’m missing debounce.
Lately I’m using Thing Status Reporting [4.0.0.0;4.9.9.9] to trigger a script, based on the ideas of Offline Things Display but enhanced by the ability to use [ Debounce [4.0.0.0;4.9.9.9]
The groupItem of Offline Things Display gets metadata “deboundUIDs”
value: <name of Debounce group>
config:
<uid of debounced thing>: <timeout>
<uid of debounced thing>: <timeout>
...
state: ON,OFF
command: false
All items are created on demand.
Thanks to the authors of the used add ons.