Hi!
couple questions about your setup:
1. Do you perform ICMP agentless monitoring for a computers where no Symantec management agent is installed? And these computers were populated on server using "Network Discovery"?
If you've used "Network discovery", then probably some servers have changed their DHCP address therefore is better to perform re-discovery task to update NIC address for all agentless machines.
2. Do you have "All instances" option in "Metric Evaluation" window?
If yes, then you should check "All instances" checkbox then in case if computer has 2 network adapters and one of them will be unreacheable but second adapter will be online, then alert will doesn't appear about that this computer is offline however it is online. Alert will appears in case if both network adapters will be offline on monitored agentless machine.
Thanks,
IP.