Not a duplicate but an alternative to Design Patterns: Generic Is Alive which doesn’t require persistence and doesn’t require polling (i.e. the every minute Rule).
The absolute simplest approach to this would be to use the Expire binding to reset your Item to UNDEF or NULL when it hasn’t received an update for too long. You can trigger a rule when those Items go to NULL/UNDEF to generate your alert. Only one alert message will be sent. See the “Expire Binding” example in the DP linked above.