This is probably caused by stale data remaining in WMI. We publish our performance data into WMI as instances, for each VM, each VM vDisk, etc. And even though the VM is shut down; sometimes the last remaining instance can be 'cached' in WMI, and so SCOM keeps collecting it.
You can see this situation in SCOM, if you look at a performance chart for a shut down VM. All the metrics will keep collecting, and the chart will be just straight lines....
And of course if the performance data is still collected, then it is possible for a monitor to keep firing, if the last performance sample was a breach of theshold. In your case, the last sample for vDisk Space was below threshold, so the vDisk Free Space monitor will keep alerting.
In our next update, we have moved to .Net 4.0 and a new WMI method, which allows us to revoke old stale data. Also our data source providers in the MP are updated, to skip powered off VMs.
For the moment in your current version, you will clear the WMI cache if you restart our nworks Virtual Infrastructure Service.
Hope that helps!
Vice President, Product Management