Card and badge readers on a building access system are network peripherals like any other, and their connectivity is worth monitoring the same way.
Some access-control panels expose SNMP for basic reachability and door status; others only offer a proprietary API or a webhook on state changes. Check your vendor's integration documentation before assuming SNMP is available.
A central access-control panel going offline is the highest-priority event; if individual door readers are separately network-addressable, monitor those too, since a single failed reader can strand people at one entrance without taking down the whole system.
A badge reader going offline during the day is inconvenient; the same event overnight or on a weekend, when it means a door may be failing open or failing locked with no one around to notice, deserves a different alert routing.