It's not actually a network discovery that is run in the background. ESX hosts and storage systems are discovered. The VMware vCenter Server provides the list of hosts, and storage systems are discovered from those hosts via SCSI VPD page 0x85 if LUNs are mounted to obtain a management network address. Storage system discovery involves invoking a number of APIs directly on the storage systems to collect information such as Vservers, vFilers, volumes, luns, aggregates, LIFs, adapters, etc. This discovery is run automatically overnight (time is configurable) and is also executed when manually invoked as you described.
There is no network discovery such as what tools like nmap provide.