Probably the easiest would be to use the expire binding to set the Item to NULL or some other noticable value (e.g. -1) after the Item hasn’t been updated for a time. It is best to set the expire time to 2x the update time so if the Xiaomi updates every 10 minutes set the expire binding to 20 minutes.
You can set a rule that triggers on changes to this item and first thing in the rule check whether it is e the expired value and alert or whatever as appropriate.
There is also [Deprecated] Design Patterns: Generic Is Alive which is a more comprehensive solution but it is a little more complicated than you need right here and I need to rewrite that DP so if you want to take that approach, wait until next week when I have a chance to rewrite it.