The most reliable way to get immediate alerts regarding the status of the nodes is to create a simple script that calls (i.e. every 120 seconds) the Bluetooth Beacon Tracker nodes API. Once you have the JSON array with all the nodes, you just have to iterate on it and check if at least one node has been seen down for more than X seconds by using a rule like the one shown in the pseudo code below:

for (i=0;i < total_number_of_nodes; i++){
if (LRRT.node(i) >= 120) send_an_alert_email_to(username@domain);
}