Under certain circumstances some or all VMs in a vSphere environment may show the red alert icon and an alert status in a VM list, but no alarm in the VM properties that could be acknowledged or cleared. These “phantom alerts” are an issue for quite some time now and still not fixed.
They seem to be caused by an interruption of the connection to the datastores these VMs reside on, for example by an ESXi host booting while the storage is not up & running. The VMs are first displayed as “disconnected”, but after the datastore connection is restored, the alert status stays.
There is no straightforward way to resolve this, but a vMotion clears the status, most likely because the VM registration is updated. So basically just move the VMs around to clear the alerts, the quickest way is to set the hosts into maintenance mode and evacuate them. A host reboot is not required.