I assume you mean orphaned VMDKs.
They happen when someone removes, but does not delete, a virtual disk from a VM. By default when you remove a virtual disk from a VM, it simply unregisters it, but leaves the VMDK on the datastore. You have to explicitly select to delete from disk to erase them from the datastore.
PowerCLI will enable you to automatically scan and report on orphaned VMDKs. There are already solutions out there for this. Example:
Discovering orphaned vmdk files in vSphere « J. Gregs Brain Corral (this script is a little outdated, but still works if you remove the line of code on line 17 "[Reflection.Assembly]::LoadWithPartialName("VMware.Vim")")
